MX records affect only to receive mail. They do not affect the sending.
The MX record is intended for specifying the address of the mail server for the domain within which it is added. You can route the MX record to an IP address or another domain. The domain to which the MX is directed must have Arecord.
For each MX record, you must specify a priority. The priority is used only if the servers are unavailable. That is, if there are two records with priority
20, then mail will initially be directed to the record with priority 10, and only if the server specified in this record is unavailable, mail will be sent to the server from the record with priority 20.