When a specific SMTP server is set on a mail template, it is never used
when the mail is sent. There are two reasons:
- `mail_server_id` is a read-only field set by an onchange. However, a
read-only field is not sent to the server, so the information is lost.
- The information is stored in the context of the object `Mail`, but it
is never used. Moreover, when the method `message_post` is called, the
`Mail` object is not used to post the message, so the information is
lost again.
The fix includes two parts:
- the `mail_server_id` field is not read-only anymore. Actually, there
is no reason to make it read-only.
- the information is stored directly in the mail values and used at the
appropriate moment.
Fixes#6554 (from v9.0)
opw-669958
Nicolas Martinelli
88e7122020