Hello,
Is your mail server having trouble resolving the MX record of .gov domains?
The .gov domains seem to have more frequent problems with DNSSEC than other TLD's.
If your mail server is using a DNS resolver that is doing DNSSEC validation that could be a reason why you can't resolve the MX record.
To figure out if that is the problem try this DNSSEC validation tool.
Is your mail server having trouble resolving the MX record of .gov domains?
The .gov domains seem to have more frequent problems with DNSSEC than other TLD's.
If your mail server is using a DNS resolver that is doing DNSSEC validation that could be a reason why you can't resolve the MX record.
To figure out if that is the problem try this DNSSEC validation tool.