Are you getting an error message whenever you try to send an email, for example via Outlook, your phone, or a webshop form? Usually the problem lies in incorrect SMTP settings, a blocked port, an incorrect password, or a server that doesn’t trust the sender. In this article, you’ll systematically go through the most common causes and solutions, so your mail gets sent out again.
What is SMTP and why does it go wrong
SMTP (Simple Mail Transfer Protocol) is the protocol that ensures outgoing email from your mail program goes to the server and from there further out into the world. When this goes wrong, it’s usually because one of these components is incorrect: the server name, the port, the security method (SSL/TLS), your login details, or a block by your internet provider or firewall.
Many people accidentally still use the incoming settings (IMAP or POP3) for outgoing mail, or vice versa. This is one of the most common mistakes and is easy to check right away.
The route of an outgoing email via SMTP
Common error messages and what they mean
Below are the most common messages you encounter with outgoing mail problems:
“Cannot connect to the server” — the server name or port is incorrect, or the port is being blocked by your network.
“Authentication failed” — your username or password is incorrect, or the account requires a different login method.
“Relaying denied” or “550 relay not permitted” — the server doesn’t recognize you as an authorized sender. This often happens when authentication is missing or incorrectly configured.
“Certificate error” — there’s a mismatch between the security method (SSL/TLS/STARTTLS) you’ve set and what the server expects.
Step-by-step plan: solving SMTP problems
Check the correct SMTP details
Look up the correct server name, port, and security method in your hosting environment or Plesk, and enter them exactly as such in your mail program.
Enable authentication
Make sure “SMTP requires authentication” is checked and use the same password as for the email account itself.
Test a different port
Some networks block default ports. Try the alternative port for a secure connection if the default port doesn’t work.
Check your password
Reset the email account’s password if you’re unsure whether it’s correct, and re-enter it manually without pasting it from an old document.
Check firewall and antivirus
Some security software blocks outgoing mail traffic by default. Temporarily disable it to test whether this is the cause.
Can’t figure it out yourself? Check the knowledge base for more explanation, or send a message to your hosting provider’s support department.
SMTP in forms and web applications
Is it not about a mail program but about a contact form or webshop that isn’t sending mail? Then the cause often lies in the configuration of the website itself. Many scripts use the server’s built-in mail function by default, but those emails end up in spam faster than when you use an SMTP connection with authentication.
In that case, check whether the form or plugin is set up to send via SMTP using the correct login details of a real email address on your domain. This also prevents emails from being seen as untrustworthy, since they then match your SPF and DKIM settings.
Want to make sure your domain is properly set up for reliable email? Then also check your verification records. You can find more explanation about this in the article on SPF, DKIM, and DMARC. For a new or additional email address, you can easily arrange this via business email hosting.
Frequently asked questions
Why does incoming mail work but outgoing mail doesn’t?
This is almost always because the SMTP settings (outgoing) differ from the IMAP or POP3 settings (incoming). Check whether you’re using the correct server, port, and authentication for outgoing mail.
Why does sending email work at home but not at the office?
Some business networks block default mail ports by default. Ask the network administrator to open the correct port or use the alternative secure port.
Can an expired password be the cause?
Yes, if the email account’s password has been changed but not updated in your mail program, authentication will keep failing until you enter it again.
Conclusion
Most SMTP problems arise from incorrect server details, missing authentication, or a blocked port. Calmly go through the step-by-step plan and check the exact error message: it usually points you directly to the solution.