[kyrra]

The "dig" command can get them for you

$ dig ycombinator.com mx

  ;; ANSWER SECTION:
  ycombinator.com. 300 IN MX 20 alt1.aspmx.l.google.com.
  ycombinator.com. 300 IN MX 10 aspmx.l.google.com.
  ycombinator.com. 300 IN MX 20 alt2.aspmx.l.google.com.
  ycombinator.com. 300 IN MX 30 aspmx4.googlemail.com.

[pixel16]

this doesnt work if they use a 3rd party email filtering service like mimecast or proofpoint fyi.

[neilv]

Another red flag! :)

[ExoticPearTree]

Proofpoint, definitely a very big red flag.