ot: selfcert IMAP StartTLS 143 Win10 Tbird woes

I was trying to set up a replacement laptop running W10/64 to replace a Win7 laptop, installed TBird, and, attempted normal setup:

IMAP StartTLS 143, SMTP StartTLS 587 (like on old laptop), and, to cut a long story, eventually discovered it won't work till I changed IMAP SSL/TLS 993 ?

SMTP seems OK, only IMAP didn't (I recall earlier similar type of issues with either Outlook or OE or WLive?)

is this the self certified part ? or what is it ?

what's the best way around ? or just leave it as 993 ?

thanks for any hints , pointers or even words of encouragement