Email warmup

SMTP warmup: how to warm up a mailbox on any host

Warmerly warms a mailbox on any host when you can give it two logins: SMTP to send and IMAP to read the inbox. A send-only SMTP relay has no inbox, so it cannot be warmed this way. Here is what to enter, which hosts are filled in for you, and what the warmup looks like on a custom server.

Updated 24 October 2026

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The short answer

Yes, Warmerly warms a mailbox connected by custom SMTP and IMAP. Choose Accounts, then Add, then Another provider. Warmerly signs in to both before it saves the mailbox.
You need both logins. SMTP sends the warmup mail. IMAP reads the inbox, so Warmerly can see replies and where each message landed. A send-only relay or sending API has no IMAP side and cannot be warmed this way.
On a custom host the same 30-day ramp applies, from 4 warmup messages a day to a ceiling of 50. Zoho is held at 25, Gmail at 40 and Outlook at 30.
Some mailboxes cannot be connected at all: Proton Mail (no direct IMAP or SMTP), a domain that only forwards mail, and a domain with no mail server.
Your host's own sending limits still apply, and Warmerly cannot read them. If your host caps outgoing mail, set a lower daily warmup limit on the mailbox.

Mailbox or relay

What people mean by SMTP warmup

The search "smtp warmup" covers two different setups, and only one of them can be warmed the way Warmerly does it.

A mailbox has an address, an inbox and a password. You can send from it and read what arrives. A relay (or a sending API) accepts mail from your software and delivers it for you. It has no inbox, so nothing can read replies. Warmup works by sending real messages between mailboxes, replying, and checking where each one landed, so it needs a mailbox on both ends.

What Warmerly can and cannot warm
What you have	Warmed by Warmerly?	Why
A mailbox on Zoho, Namecheap Private Email, Fasthosts, Titan, IONOS, Hostinger, Fastmail, Rackspace, OVH, Yandex or your own server	Yes, by SMTP and IMAP	It has an address to send from and an inbox to read.
Gmail or Google Workspace	Yes, with an app password	Google asks for a special password for this kind of connection. See Gmail warmup.
Microsoft 365, Outlook.com or Hotmail	Yes, by Microsoft sign-in	Sending needs Authenticated SMTP switched on for the mailbox. See Outlook warmup.
An SMTP relay or sending API, such as Amazon SES, SendGrid, Mailgun or Postmark	No	No inbox, so there is no IMAP login to give. Warmerly needs both sides.
A domain that only forwards incoming mail	No	There is no mailbox behind it to send from. Connect the mailbox the forwarding delivers to.
Proton Mail	No	Proton does not expose IMAP or SMTP directly. It only works through Proton Bridge on a desktop machine, which Warmerly cannot reach.
A domain with no mail server in DNS	No	There is no mailbox to connect. Check the address, or set up mail on the domain first.
A personal @gmail.com address	Not supported for cold email	Warmerly does not support personal Gmail for cold email. Use a mailbox on your own domain.

Before you start

What to have ready

The SMTP server: host name, port, username and password. This is the outgoing mail setting.
The IMAP server: host name and port, and a separate username and password only if your provider issues a different login for reading mail. Warmerly has a checkbox for that case: "My IMAP (incoming) login differs from my SMTP (outgoing) login".
A public host name. Warmerly refuses private, local and internal addresses, because the connection is opened from our servers. Use your provider's public name, for example smtp.example.com.
An app password if your provider uses two-step sign-in. Many providers refuse the normal password for this kind of connection. Create the app password in the provider's security settings and paste that.
Published authentication for the sending domain. Warmup builds a sending history. It cannot repair a missing SPF, DKIM or DMARC record. Check them with the deliverability checker first, and read the cold email deliverability checklist if the domain is new.

Almost every provider publishes its SMTP and IMAP settings on its support site. Search for the provider's name and "SMTP IMAP settings". If the provider is in the table further down, Warmerly fills the values in for you.

Ports and encryption

Which ports to use

Warmerly treats SMTP port 465 and IMAP port 993 as encrypted from the first byte, and most presets use them. RFC 8314 (January 2018, read at rfc-editor.org on 24 October 2026) recommends exactly that: TLS from the start of the connection, with 465 as the default port for SMTP submission and 993 for IMAP. A few hosts use 587 for SMTP, and Warmerly's iCloud preset does.

Warmerly does not restrict you to a list of ports, because real hosts run mail on unusual ones. The host name must be public, and the connection test decides whether the login works on the port you chose. If the test reports a secure-connection problem, the port is usually the wrong one for that host's encryption type. Try 465 or 587 for SMTP.

Settings filled in for you

Hosts Warmerly recognises

When you type an address, Warmerly looks up the domain's mail records and, for hosts it knows, fills in the server settings and hides the server panel. You then enter only the password. These are the hosts and settings it uses today:

SMTP and IMAP settings Warmerly fills in for recognised hosts
Host	SMTP (sending)	IMAP (reading)	Note
Zoho Mail	`smtp.zoho.com` : 465	`imap.zoho.com` : 993	App-specific password if two-factor is on
Namecheap Private Email	`mail.privateemail.com` : 465	`mail.privateemail.com` : 993	None
GoDaddy (managed Microsoft 365)	`smtpout.secureserver.net` : 465	`imap.secureserver.net` : 993	None
Titan Mail	`smtp.titan.email` : 465	`imap.titan.email` : 993	None
Fasthosts	`smtp.livemail.co.uk` : 465	`mail.livemail.co.uk` : 993	None
Spacemail	`mail.spacemail.com` : 465	`mail.spacemail.com` : 993	None
Hostinger	`smtp.hostinger.com` : 465	`imap.hostinger.com` : 993	None
Fastmail	`smtp.fastmail.com` : 465	`imap.fastmail.com` : 993	App password with mail access
IONOS	`smtp.ionos.com` : 465	`imap.ionos.com` : 993	None
Rackspace Email	`secure.emailsrvr.com` : 465	`secure.emailsrvr.com` : 993	None
OVH	`ssl0.ovh.net` : 465	`ssl0.ovh.net` : 993	None
Yandex Mail	`smtp.yandex.com` : 465	`imap.yandex.com` : 993	None
Yahoo Mail	`smtp.mail.yahoo.com` : 465	`imap.mail.yahoo.com` : 993	App password required
iCloud Mail	`smtp.mail.me.com` : 587	`imap.mail.me.com` : 993	App-specific password required

Gmail and Google Workspace are filled in too (smtp.gmail.com on 465 and imap.gmail.com on 993), with an app password. Microsoft 365 and Outlook.com use sign-in rather than a password.

For a host Warmerly does not recognise, it guesses mail.<your domain> on 465 and 993, opens the server panel so you can correct it, and lets the connection test decide. If the DNS lookup does not answer in time, you fill the settings in yourself.

Step by step

Connect the mailbox and start warmup

Check the login works elsewhere. Sign in to the provider's webmail, or to any mail app, with the same SMTP and IMAP settings. If that fails, Warmerly will fail too.
Publish SPF, DKIM and DMARC for the sending domain and check them with the free SPF checker, DKIM checker and DMARC checker.
Open Accounts and choose Add. Pick Another provider and type the mailbox's address.
Check the settings. For a recognised host they are filled in. Otherwise open the server panel and enter the SMTP and IMAP host and port. Tick the box for a separate IMAP login if your provider needs one.
Enter the password, or the app password if the provider uses two-step sign-in.
Choose Connect and start warmup. Warmerly tries SMTP and IMAP and saves the mailbox only if both work. If one fails, you see a Sending (SMTP) row and a Reading replies (IMAP) row, so you know which to fix.
Leave it running. Do not send campaign mail yet. See how long email warmup takes for the signals that say a mailbox is ready.

To re-run the check later, open the mailbox and choose Test connection from the menu in its header. Warmerly shows the plain-language reason and the raw reply from your mail server, including the exact SMTP code, which is what support needs.

What warmup does on a custom host

The ramp on a custom SMTP mailbox

A mailbox connected by custom SMTP and IMAP follows the same fixed ramp as any other, counted in days the mailbox has actually warmed. Hosts Warmerly does not name as Gmail, Outlook, Zoho or Namecheap are stored as custom, which has a ceiling of 50 warmup messages a day.

Warmup messages a day on a healthy weekday
Ramp day	Custom host	Zoho
Day 0	4	4
Day 3	8	8
Day 7	15	15
Day 14	25	25
Day 21	35	25
Day 30	50	25

A custom host reaches 50 a day on day 30. Zoho is held at 25 from day 14. Weekends run at 60% of a weekday, and the day's volume is cut back if the mailbox's health score slips. Warmup messages go to other mailboxes in the warmup network, spread across providers, and are read and replied to over IMAP.

Your host may be stricter than Warmerly

Warmerly's ceiling is Warmerly's. It cannot read your host's own limits on outgoing mail, which differ between providers and often between plans. If your host caps messages per hour or per day, set a lower daily warmup limit on the mailbox.

If a host keeps refusing sends or logins, Warmerly cuts the mailbox back to a single send a day while it fails and, after repeated refusals, pauses it and asks you to reconnect. A wrong password is retried for a limited time, so a changed password needs a reconnect.

When it will not connect

What each message means

Connection messages and the usual fix
You see	What it means	What to do
Wrong username or password	The login was refused.	Recheck the address and password. If the mailbox has two-step sign-in, it needs an app password.
Needs an app password, not the password you sign in with	Two-step sign-in is on.	Create an app password in the provider's security settings and paste that.
The provider blocked this sign-in as unusual	The provider wants you to confirm it was you.	Sign in to webmail in a browser, confirm, then try again.
Couldn't reach the mail server	The host or port is wrong, or the server is down.	Check the host and port against the provider's own page.
Secure-connection problem, the port may be wrong	The port does not match the encryption type.	Try 465 or 587 for SMTP.
The provider is temporarily blocking logins	Too many attempts in a short time.	Wait a few minutes and try again.
Private, local and internal addresses are not allowed	The host name resolves to a private address.	Use the provider's public host name.

The longer version, with provider-specific fixes, is in the docs: My mailbox won't connect and Connecting a mailbox.

If you came for an SMTP relay

Relays, sending APIs and warmup

An SMTP relay accepts mail from your application and delivers it. Its job is delivery, not conversation, so it has no inbox to read and nothing to reply with. That is why Warmerly's mailbox warmup does not apply to it.

A relay sender still builds a reputation, and the same advice holds: authenticate the sending domain, start with a low volume, raise it slowly, and watch bounces and complaints. See what an SMTP relay is for how relays work, and the cold email deliverability checklist for the rules the big mailbox providers publish.

If what you need is to send application email through an API or SMTP, Warmerly's transactional email is a separate product for that. It is not part of mailbox warmup.

Control and security

Who can see the password, and how to cut the connection

Warmerly stores the SMTP and IMAP passwords encrypted.
To cut the connection from the provider's side, change the password, or delete the app password at the provider.
Warmerly tests both logins before it saves anything, so a mailbox is never saved that could not sign in once.
For a Microsoft 365 or Gmail mailbox, use the one-click sign-in or the app password. Do not reuse the normal account password.

Sources

Where the outside figures come from

Every figure from another company on this page was read from that company's own page on the date shown. Pages change, so check the source before you rely on a number.

RFC 8314, Cleartext Considered Obsolete: Use of TLS for Email Submission and Access (January 2018), read 24 October 2026

Questions

Can I warm up a mailbox that I connect by SMTP and IMAP?

Yes. Choose Accounts, then Add, then Another provider, and give Warmerly the SMTP and IMAP details of the mailbox. It signs in to both before it saves the mailbox and then starts the warmup ramp.

Do I need IMAP as well as SMTP?

Yes. SMTP sends the warmup messages. IMAP lets Warmerly read the inbox, so it can see replies and where each message landed. Sending without reading would mean no replies and no placement data, so the connection test checks both.

Can I warm up an SMTP relay such as SES, SendGrid or Mailgun?

Not with mailbox warmup. A relay or sending API has no inbox, so there is no IMAP login to connect. Authenticate the sending domain, start with a low volume and raise it slowly instead.

Which port should I use for SMTP?

Use the provider's published settings. Warmerly treats port 465 as encrypted from the first byte, and RFC 8314 recommends that style of connection. Some hosts use 587. IMAP normally uses 993.

How long does warmup take on a custom SMTP mailbox?

Warmerly's ramp runs 30 days, from 4 warmup messages a day to 50. A custom host reaches 50 on day 30. Readiness is judged on signals such as authentication passing and a steady health score, not only on the calendar.

Why does my mail host refuse the login?

Usually the password is the normal one and the host wants an app password, the host or port is wrong, or the provider has blocked the sign-in as unusual. The mailbox's connection test shows separate Sending and Reading rows, so you can see which side to fix.

Does it work with my own mail server?

Warmerly connects over SMTP and IMAP, so a server that offers both on a public host name, with a login, can be connected. Warmerly does not publish a tested list of server software, and running your own server means reverse DNS and TLS are yours to maintain.

Is the warmup ceiling the same on every host?

No. A custom host has a ceiling of 50 a day, Gmail 40, Outlook 30 and Zoho 25. Your host's own limits on outgoing mail may be lower, and Warmerly cannot read them, so set a lower daily warmup limit on the mailbox if you need one.

Can I warm up a Proton Mail address?

No. Proton does not expose IMAP or SMTP directly. It only works through Proton Bridge on a desktop computer, which Warmerly cannot reach. Use a different mailbox for warmup and outreach.

Connect any mailbox and start the ramp.

Add a mailbox by SMTP and IMAP, and Warmerly tests both logins before it starts warming. Start free.

Start free See pricing

Free plan, no time limit
7-day trial on paid plans
30-day inbox guarantee, with conditions
Customer data is hosted in the EU