Can't send any emails from my iMac running macOS 15.5

On my iMac, I can receive emails but I am totally unable to send any emails.

My email settings and password are correct as I can also send and receive emails while using web mail with my ISP.

My OS 15.5 and my email version is 16.

I am at a total loss and any help would be appreciated.

[Re-Titled by Moderator]

Original Title: Can't send any emails