[FIX] mail: Ignore case for mail aliases.

When receiving new mail (not replies) to an alias we should not take
case into account.

This also homogenize the treatment of local parts.  For instance, lines
967 and 980 convert the local part to lower case to avoid
case-sensitivity issues.

Also `mail_alias` normalizes alias names by lowering case and finding,
if necessary to make it unique, a suffix to alias name provided.

From RFC 5321, section 2.4:

  > Exploiting the case sensitivity of mailbox local-parts impedes
  > interoperability and is discouraged

Closes #334
Closes #13037

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