I have typically used external email servers with odoo but with odoo.sh, the literature states, 

"You have nothing to do! Odoo sets up its own mail servers for your database. Outgoing and incoming emails work out-of-the-box!"

So i thought i would try this but information is thin on the ground. From what i can tell, catch all emails work  (using the odoo.sh domain   ie    catchall@yourdomain.odoo.sh )

And you can change that to just catchall@yourdomain.com   with a mx record redirection and changing mail.catchall.domain  - however the million dollar question is .... if you have a user with an email sales@yourdomain.com  can they receive an email into odoo.sh (that wasnt sent in reply to an email that originated in odoo (send via a catchall email). I cant seem to get that to work (i do a mx record redirect but still nothing).

Secondly, does odoo.sh automatically create SPF-compliant and dkim records (i dont think so because they need to be done on your dns records. 

I will probably go back to external servers but it would be nice if i dint have to.