mail.fortress.org has no MX records.
Question and response
Question
dig mail.fortress.org. MX
Answer
The response has no answer records.
Authority
fortress.org. 3600 SOA ns.pubnix.net. purple.fortress.org. 2025010501 10800 300 2419200 43200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0