Mail was the suffix given to earlier long run express trains meant for postal transferring coaches, apart fom pasenger boggies, mostly pre-indepedence, as nowdays o new trainnamed as mail.
These trains were prioritized high because of postal urgencies.
Express are the same suffix given to trains in recent time, with less difference.
Now postal service doesnt depend on Mail/Express suffix, as Punjab Mail no...
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more have RAIL MAIL coach, while HWH ASR express do have.
Agan now, mail are supposed to be slower compared to SUPEFAST OR EXPRESS, cz they are too old with lots of induced stoppages & if the mail isnt superfast, then its worthless (Barauni mail)

Please see #22234 : copied here.. back to questioning GK about Railways Mr Krish !!?? Only minor difference from the other answer about clearances earlier for MAIL trains: Kalka Mail, Frontier Mail, Gujarat Mail, Bombay Mail, Howrah Mail etc.
This naming of a train as a MAIL TRAIN originates from the history of Indian Railways. Earlier, trains would carry mail (letters, post cards, etc. i.e. Postal Department) in a separate coach earmarked specially for the the Postal Department. Since the train would carry "Mail" it would have greater clearances on routes where it operated over other trains, just as today's Shatabdis/Rajdhanis get clearance.
Today...
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also these trains carry MAIL ALTHOUGH the separate coach may not carry the marking as Postal Coach as earlier. However, an unreserved coach is marked as "for Postal Department only" or some such indication.
Therefore, the name persists. It has NOTHING to do with slip coaches or other facets highlighted by bloggers. That is part of the system. But I hope this clarifies why trains WERE/are still called Mails.....
Today even ordinary trains like West Coast Express carry "postal department coach" (I have seen this personally !), but it is NOT called MAIL !!!
Good wishes. And congratulations to the other person who answered this. Note only the precedence given to Mails...then, and now..