According to the proximus.be site, the recommended settings are:

incoming IMAP server: imap.proximus.be, port 993, SSL/TLS security, authentication = normal password, User Name = email address

outgoing SMTP server: relay.proximus.be, port 587, STARTTLS security, authentication = normal password, User Name = email address

Check that you have these in Tools/Account Settings, Server Settings, for the incoming and Outgoing Server (SMTP) for the outgoing.

If you have all that and it still won't send, it might be your antivirus blocking the connection, especially with an app like AVG/Avast, Norton etc.

I checked the settings in the account settings and they were correct. I uninstalled Avast but that could not solve the issue. Proximus told me to leave Thunderbird and to switch to Proximus email which I will not do for the moment. I hope that somebody can solve this problem with the outgoing mails.

If you removed (not disabled) Avast and rebooted the PC, I would expect the account to work, unless there are some other 3rd-party security apps running in the background, or you use a VPN. Any error messages appearing when sending? If you have multiple accounts, is each one sending on an smtp with a matching User Name? Select an account in the left pane of Account Settings, check Outgoing Server (SMTP) in the right pane and click Edit SMTP server... to view the settings.

Thanks, but I did remove the Avast and rebooted the PC but the problem remains. Is this an unsolvable problem, it looks like. What can we do more?

Perhaps fixed by bug 1854567 in 115.6.0, available next week. Please post your results when using that version.

Hi, the problem of the outgoing mails remains also with the 115.6.0 version.