Change Message-ID to be more unique and leak less information
A Message-ID should be globally unique. Currently mutt generates this ID based on the current date and time, followed by ".G", followed by a letter A to Z (A for the 1st and 27th email sent, Z for the 26th, etc.), followed by the pid of the active mutt process, followed by "@" and the configured fqdn.
This can lead to information being leaked as to an users email habits and activities, which might be undesirable.
By replacing everything left of the "@" in the Message-ID with a Base64 encoded timestamp and 64 bits of randomness, we no longer include this information.