Mail intermittently stops receiving email

Feb 23rd 2019

I'm running MacOS 10.13.6 and Mail 11.5

A few months back Mail started stopping intermittently for days at a time and then for no apparent reason would download the backlog of mails until it all happened again, usually some weeks later, but sometimes just a few hours later.

I have gone to Mail>Take all account Offline, and then Mail>Take all accounts Online, but that doesn't help.

I have restarted Mail, restarted the iMac to no avail.

I can get web mail which makes clear that there is mail ready to download.

Thanks.