Check whether Avast is blocking SMTP sending in Outlook or Thunderbird

If Outlook or Thunderbird stops sending and shows a timeout, Avast is one possible cause you need to weigh against others, not an automatic conclusion. For example:

El mensaje no se ha podido enviar porque ha caducado la conexión al servidor SMTP

(In English: "The message could not be sent because the connection to the SMTP server timed out".)

The timeout doesn't identify the cause. First check that the server, port, encryption, authentication and username match cPanel > Email Accounts > Connect Devices. Also try Webmail and, if possible, another network to tell a server problem apart from a local one.

If the fault started right after installing or updating Avast, its mail filter becomes a hypothesis you can check with a short test.

Go to Menu > Settings > Protection > Core Shields > Mail Shield.

Once there, turn off Scan outbound emails (SMTP) for a single send only and turn it back on straight away. If you need to isolate it further, run one more single test with Mail Shield turned off, turn it back on when you finish and don't lower the SMTP encryption. If sending works only during these tests, keep protection on and review Avast's installation, updates, inspection certificate and settings. If it persists, use Avast's support to resolve the mail inspection.

If the result doesn't change, Avast isn't confirmed as the cause. Keep the full error and the time so the connection and SMTP logs can be checked.

Menu names depend on the version and language. The official guide to Avast's shields helps you find the current setting. A test from Webmail uses a different connection and doesn't confirm on its own that the computer can reach the SMTP port.