Mail - keeps shutting down as soon as I open it

When launching mail on my iMac it opens, tries to connect & then shuts down again, all very quickly. Any ideas of where to look please. I've deleted all new messages off of my incoming accounts, Cloud, Gmail & Yahoo, cleared out the SPAM folders in each & the trash folders in each, so basically no new messages to import from the last time the Mail App was opened.

I have an iMac (Retina 5K, 27-inch, Late 2014 and all software is up to date.