This question was originally asked in 2014 by someone else.
I can get the Shared Mailbox to work in OTHER IMAP clients without issue, using the " ActualUsername backslash sharedmailbox alias" method. (e.g. firstname.lastname@example.org\support)
SMTP sending is working fine via Office 365, and non-shared mailboxes are all working fine in SugarCRM.
It just appears that the backslash in the username is maybe screwing up the authentication username.
Is Sugar replacing backslashes? or stripping them? Doing double backslash didnt work either.