This happened to us too, but only for one domain so far.

I saw it on at least 6-7 of my domains.  All since early January.

601 errors are generally related to network issues. 

The connection could have been interrupted in transit, the connection could have been dropped, or timeout out. The best way to test this is to run a telnet command from your server to the IP addresses that are in the 601 error. 

After making the connection enter a EHLO command and check the response. If you get something other than a 220 response when making the original connection or a similar reply to the EHLO command you will need to further troubleshoot the issue. 

A network package sniffer can help with this. Wireshark, Fiddler.

Thanks Tony, I'll give that a try.

Guys, I have a customer with the same issue on SM 15x. Tony sent me something today that may be useful to you guys if any of you have a Cisco router in front of your server. He said that if you do, it may be causing the 601 issue. He provided this fix just in case... https://www.tunnelsup.com/mx-record-problem-reverse-dns-does-not-match-smtp-banner/

I'm still waiting to hear back from my customer as to whether or not he has a Cisco firewall, so I have not tested this personally. I was just passing it along.

I used telnet and was able to connect to 5 out of the 6 I tested.  The one that seemed questionable, I wasn't able to ping the domain at all.

Of the 5 success, 4 responded immediately with a 220.  

The other was blank, but then when I tried to type something else into the box, I got a 220 barracuda.bdlaraway.com.

The client that started me investigating owns 2 of the 6 I'm testing.  He indicated that he used to be able to email these clients without issue.  I confirmed that as of 12/20/18, my client could send mail to both of these domains without error.

None of our hardware has been changed recently.  I'll have to check to see if there are any firewall changes.