resolving addresses in smtpd and T-mobile addresses I login on with SSH being used

I have been fixing some problems with emails between two of my servers.
At the time I had /etc/hosts file empty except for localhost.

While sending emails, I got several of errors in maillog like these:

Dec 2 22:36:28 freedomforlife smtpd[78001]: cd3e9bc4ab696630 smtp connected address=172.58.46.253 host=<unknown>
Dec 2 22:36:28 freedomforlife smtpd[78001]: cd3e9bc4ab696630 smtp failed-command command="" result="550 no rDNS is so 80s"
Dec 2 22:36:28 freedomforlife smtpd[78001]: cd3e9bc4ab696630 smtp disconnected reason=quit

172.58.46.253 is an IP for T-mobile. Which I use the WiFi hotspot to use SSH from.
This was a previous IP I had used before, not on the session in progress.
I found several other T-Mobile addresses being used in maillog that were not current sessions.

I don't know what to make of this. I rebooted both servers after emptying /etc/hosts.

resolv.conf has:

lookup file bind
nameserver 8.8.8.8
nameserver 8.8.4.4

on this server.

What is going on here? How could this possibly be happening?

Chris Bennett