SMTP Relay not sending: connection, TLS, login, sender and recipient problems, one by one

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The short answer: nearly every problem falls into one of five drawers: cannot connect to the server, TLS error, login refused, sender refused or the recipient gets nothing. The error message in your program tells you which one. The table below says what to do in each case.

Symptom, likely cause and fix

Symptom Likely cause What to do
Cannot connect (hangs, «timeout», «connection refused») The port is blocked by the network or by the hosting provider where the program runs. Try the other port: 587 or 465. On a VPS, check the firewall too.
TLS or certificate error Port and encryption swapped. Sometimes an old program or device, or one with the wrong clock. 587 with STARTTLS, 465 with TLS. Do not mix them. Update the program and fix the clock.
Login refused Wrong username or password, or a revoked credential. Create a new credential in the Email Center and copy the password carefully.
Sender refused The address is on a domain you have not verified. Verify the domain or use an address on it.
The recipient gets nothing The address is on the blocked list, or the e-mail went to spam. Check the log: it shows the status of every e-mail.

The codes the server returns

When the program shows a three-digit number, that is the SMTP server reply. The general rule of the protocol helps you decide what to do:

Starts with Means What to do
2 Accepted. The e-mail is in the queue. Nothing. Follow it in the log.
4 Temporary refusal. Try again later. Most programs do this on their own.
5 Permanent refusal. Repeating will not help: fix it first (credential, sender, recipient).

Where to start

1 Copy the exact error message from the program or its log. It nearly always holds the answer.
2 Test the credential outside the program, from the same machine: see testing a credential from the command line. If the test works and the program does not, the problem is in the program settings.
3 Open the Email Center log. If the e-mail is there, the relay accepted it, and what is left to check is the delivery: finding the e-mail in the log.
It worked and suddenly stopped? Think about what changed: someone revoked the credential, the program was updated and lost its settings, or the hosting provider where the program runs closed its outbound ports. On one of our VPS, see ports and firewall on a VPS.
It arrives, but in spam? Then it is no longer the relay: it is SPF, DKIM, DMARC or the content itself. Start with SPF, DKIM and DMARC.

Still not sending? Open a ticket with the exact error message and the time of the attempt, without the password.

Open a support ticket

SEE ALSO

Testing an SMTP credential before going live: a test e-mail from the command line

The program says sent, the customer got nothing: finding SMTP e-mails in the Email Center log

SMTP Relay in any program: what Host, Port, Encryption, Username and From mean on each screen

SMTP ports 465, 587 and 25: SSL, STARTTLS and which one to use

Which ports are open, and why your integration hangs without an error

Bounce message 550: user unknown, relay denied and rejected, what each one means

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