is an A record that points to 126.96.36.199 (your web server's IP address) then you need an MX record for polistem.org
that points to mail.polistem.org. Finally, mail.polistem.org
is an A record that points to 188.8.131.52 (your mail server's IP address). MX records should point to A records.
DNS RR setup:
mail.polistem.org A 184.108.40.206
polistem.org MX mail.polistem.org
Don't forget to include the priority for the MX record but I think you can figure that out.
Look at your current records, it seems you have this set up already as polistem.org points to a different IP than the MX record mentions. NOTE AGAIN, an MX record should not point to an IP address.