Perform some code cleanup not really related to other commits notably in mail
gateway where alias domains will have some impact. Extract some processing
in sub-methods, allowing to better distinguish code purpose. This implies
notably some checks in mail gateway (write to bounce or catchall detection).
In mail.message, reorder some fields according to their usage, just to keep
definitions / section.
Also improve docstrings and/or fix some of them.
This is mainly a "reduce diff in other commits" commit. No change should occur
with this commit.
Task-36879 (Mail: Support Multi Domains Aliases)
Part-of: odoo/odoo#76734
Thibault Delavallée
c53aae845d