Send File Doesn't Work

Up until recently, when I clicked Send File in Adobe Acrobat DC, my PDF would automatically appear as an attachment to a blank email in Apple Mail. Recently what now happens is Acrobat asks for, and then rejects, IMAP credentials to my outgoing server. I have properly set up my email credentials via Preferences > Email, but Acrobat refuses to accept these credentials.

I'm baffled (and very annoyed) at this recent behavior. I've even tried to uninstall/reinstall Acrobat DC, to no avail.

Has anyone else experienced this problem? Can anyone offer a solid fix?

Scott