Storing mail in database, data type
First, find out if "mail" guarantees that all data is utf8 or ascii or whatever.  If utf8, then use

MEDIUMTEXT CHARACTER SET utf8mb4


for a limit of 16MB.  For a 4GB limit, change to LONGTEXT.

If the mail is not forced into some character encoding, then use

MEDIUMBLOB or LONGBLOB


(without a charset).