It looks like their email server is down, I'm having the same issue I think:
[000.015] MX(10) 2fa.warframe.com
[000.019] MX:A-->2fa.warframe.com unresolvable
[000.000] Trying TLS on 2fa.warframe.com[unresolvable:25] (10)
[000.000] Cannot connect to server (reason: host is unresolvable)
postfix gave me this message when I tried to email no-reply@2fa.warframe.com: Host or domain name not found. Name service error for name=2fa.warframe.com type=AAAA: Host found but no data record of requested type