My emails often rejected as 'spam'

Try going to Applications/Utilities/Keychain Access and delete the certificate associated with your provider. Restart the computer and try Mail again. This should cause an updated certificate to be downloaded.

Based on your screen captures your problem seems to be mainly to do with SMTP i.e. sending emails. Some history about SMTP will be useful I think for you to understand the possible cause and what to do about it.

Originally - back in the dawn of time 😉 SMTP was a completely open unsecured process, your client program would use SMTP to talk to your server without the need for any user name or password and similarly your server would talk to the other servers via SMTP again without any security measures. Then we entered the dark ages of Spam, Spam, and more Spam. 😟 Spammers realising that there was no security at all on SMTP were able to send vast quantities to all mail servers and pretend to be anyone. In order to try and at least reduce the amount of spam hitting their servers while still retaining compatibility with the majority of other mail servers, the rules were changed. It was now necessary for client software to use a username and password when sending emails to a mail server via SMTP but server to server SMTP still does not need this. A further step in order to help differentiate between user to server and server to server SMTP traffic was that users i.e. email clients were moved to a different network port number. Server to server SMTP still uses port 25 but user to server most often these days uses port 587. A second reason to use a different port number was that by this time many Internet providers disallowed the great unwashed on the Internet from using port 25 at all as a further measure to try and block spammers who had taken to 'infecting' hordes of Windows PCs and using them as zombies to send their filthy product.

These measures have helped a little but not completely as spammers still find other ways to send their messages. Spammers still rely on infecting PCs but now often as part of this install complete mail server programs to let them pretend to be a mail server so as to send out vast quantities of spam.

In more modern times another issue not directly related to spam has arisen which is privacy. For this people quite rightly want to stop people from being able to read their messages. In order to do this the exchange of messages between a user and a mail server is now ideally encrypted using SSL. This however typically also requires using yet another different network port for both IMAP and SMTP. This means SMTP with SSL might now typically use port 465.

So your email client might need to use - port 25 or port 587 or port 465 or even some other number if your administrator has chosen a unique number. Apple try and be helpful by offering an 'Automatically detect and maintain account settings' option which tries to pick automatically the right port number. Most of the time this works ok but sometimes it does not, particularly if your using a laptop and travelling which means you might end up using different Internet providers some of which may block certain ports.

Therefore the moral of the story 😉 is to possibly consider turning this auto function off and setting it to a specific port number that you know is correct for use with your mail server.

(Measures to help stop spammers pretending to be you is another storey for another day. 🙂)

eloisemac wrote:

Thanks for the really clear explanation about what SMTP is - was completely clueless before!

I was given a specific port number to use with the mail server but it didn't help the matter. Have no trouble at all with other email clients, or with Mail on OSX so I guess it must be something to do with Mail itself.

Yes but if you leave the auto option ticked Mail might try itself to change the port number. Have you unticked the auto option?