SMTP authentication error in Outlook or Thunderbird

Reviewed on September 8, 2026 3 min read autenticacion clientes de correo

If the error shows 535 or authentication failed, the server is rejecting authentication. Keep the full code and text: relay denied, a timeout or a certificate error each need different checks.

Check the mailbox details

In cPanel > Email Accounts > Connect Devices, look up the connection details for the affected account:

Setting	What to check
SMTP server	The name cPanel shows, covered by its certificate.
Port and encryption	The secure combination shown, usually 465 with SSL/TLS; 587 with STARTTLS if it is listed among the supported options.
Username	The mailbox's full address, for example `correo@tudominio.com`.
Password	The mailbox password, which may differ from your cPanel or client area password.
Authentication	Turned on for the outgoing server.

Try logging in to Webmail with that account to check its credentials. Being able to receive email in the client doesn't prove that SMTP has the same password saved.

Outlook

Open the affected account's settings and review the outgoing (SMTP) server options. Where they are differs between classic Outlook, the new Outlook and the Mac versions; the old "Servers" tab doesn't appear in all of them.

In versions that offer My outgoing server (SMTP) requires authentication, turn that option on. Use the same settings as the incoming server only if both really use the same username and password. If there are several accounts, check that you are editing the one shown in the From field of the failed message.

Thunderbird

In Account Settings, check which Outgoing Server (SMTP) is assigned to that mailbox. Edit that entry in the list of SMTP servers and apply the cPanel details. For a Hostsuar account that uses a password, select Normal password together with the TLS encryption shown: the name of that method doesn't mean you should send without encryption.

Don't change a shared SMTP entry without checking which other accounts use it. A definition should only be shared when server, port, encryption and username all match.

Check the result

Save the changes and send a single test message. If the program asks for the password again, enter the current mailbox password. Don't try passwords over and over, and don't use ports 25 or 26 without encryption to work around the failure.

If the rejection continues, tell support the program and its version, server, port, encryption, time and exact error, without including the password. That way we can tell incorrect credentials apart from a restriction or a client fault.