Their MX server (SMTP) is:
natmail.natnet.com
Code:
; <<>> DiG 9.8.3-P1 <<>> mynatnet.com mx
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13114
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;mynatnet.com. IN MX
;; ANSWER SECTION:
mynatnet.com. 900 IN MX 10 natmail.natnet.com.