generateMessageID: appended the substring following the last '@'
without sanitizing it, so a sender value such as 'Doe, John <a@b>'
produced 'Message-Id: <uuid@b>>' with a duplicated closing bracket on
calendar invitations. A nil domain also left the message-id
unterminated.
The domain part is now cut at the first address delimiter or
whitespace - a domain is a single token, and anything else would
allow header injection through generated message-ids.
Fixes#6201
