Add support for a standard way to handle OOF auto reply

[h-coudrais]

We're delivering a lot of mail, something like 160.000 per month, and sometimes, users are "Out of office" for vacation.
When we're in vacation periods, we receive tons of mail saying "I'm in vacation"... and this is annoying :).

I assume this is a common issue you have to deal with, this is why I'm asking that:
It would be great to have an option that automatically include the correct headers to avoid a reply when users are in vacation.

As far as we know, it exists these headers:

• Precedence: bulk
• X-Precedence: bulk
• X-Autoreply: yes
• Auto-Submitted: auto-generated
• X-Auto-Response-Suppress: all
  But some headers lead to be seen as spammers by some anti-spam mechanisms.

I performed ​​some tests by playing with these different headers. I only kept Precedence: bulk and Auto-Submitted: auto-generated because the other had not the targeted effect.

Here is a draft that brings to swiftmailer a standard way to solve this issue.

[xdecock]

Should be added in documentation i think, not a big fan of "hardcoding" a solution, email is unfortunately a complex matter as it will probably be overused by users without tought of the impact (if for example you send invoices automatically, you'll want to still receives ooo to forward the invoice manually to someone else.

Just toughts however, but the system indeed has some uses.

The best way to do this at your level is to extends message with those defaults in construction phase. I fear the SimpleMessage will become a SimpleMess if we had a lot of "shortpath"

[stof]

IMO, this is best handled through a Swiftmailer plugin

[fabpot]

I agree with @stof that this should be moved to a plugin instead and docs should be updated accordingly. @h-coudrais Can you do these changes? Thanks.