Your host
Blame your host... . looks like they are blocking mail from TDR:
maillog:Mar 5 17:37:31 linux1788 sendmail[21459]: g25MbQt21457: to=radixr@velocity.net, ctladdr=nobody (99/99), delay=00:00:05, xde
lay=00:00:05, mailer=esmtp, pri=30931, relay=mx. velocity.net. [208. 3. 88. 3], dsn=5. 0. 0, stat=Service unavailable
And a prior log entry:
maillog. 5:Feb 28 05:04:49 linux1788 sendmail[12059]: g1QMMMe13281: to=radixr@velocity.net, ctladdr=nobody (99/99), delay=1+11:42:27,
xdelay=00:00:05, mailer=esmtp, pri=2640898, relay=mx. velocity.net. [208. 3. 88. 3], dsn=4. 2. 0, stat=Deferred: 450 Client host rejected
: cannot find your hostname, [128. 242. 197. 111]