Mail malfuntion
"Login failed "
message ON IMAC plus flashing Compose screen even on exit of Mail.
It seems to be ok on ipad and iphone
iMac 27", macOS 10.13
Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?
Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.
Also, have you changed your email password lately?
In the interim use a browser to login to your WebMail.