First you can check that the declaration is correct, using dig:
# 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 69.89.31.56
;; 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.
