First you can check that the declaration is correct, using
# dig foobar.com MX ; <<>> DiG 9.7.3 <<>> foobar.com MX ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 32034 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1 ;; QUESTION SECTION: ;foobar.com. IN MX ;; ANSWER SECTION: foobar.com. 14400 IN MX 0 foobar.com. ;; ADDITIONAL SECTION: foobar.com. 14400 IN A 18.104.22.168 ;; Query time: 198 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Fri Jun 23 18:56:51 2017 ;; MSG SIZE rcvd: 60
Next you can use an online tool, like mxtoolbox to check others requirements and make sure the server's IP isn't already included into a commonly use blacklist.