MacMail crashing after sending email

[#1]

Clutching at straws here, as mail is not one of my specialities, but I'm wondering if the fact that your outgoing port is not the correct one - it should be 587 - has any bearing on this? 

[#2]

-- host:relay.plus.net -- port:25 --

Use of SMTP Port 25 often means encryption is disabled causing the Credentials and message contents to be exposed.

[#3]

It is not the correct port anyway, it is only meant for MTA to MTA  transmission. 

[#4]

I think I might go for an operating system upgrade as that will (or strictly should) update Mail.

My guess is that it’s a problem with Mail.

Alternatively try a different Mac mail client.

[#5]

Is Thunderbird email client an option for you ?

[#6]

Thinking about it further, my first move would be to create another account on the Mac and then add the mail account (as IMAP) to that.

If that works ok it suggests something about the first account being a problem. If not it eliminates that possibility.