What version of SmarterMail are you running?  Do you have any configured SMTP blocks?

Build 7242 (Oct 30, 2019 

I have a half dozen or so SMTP blocks - but nothing related to this domain.

I had a problem, not sure if it is similar, but it was related to TLS.  No info in the logs saying why, but it acted similar.  It was actually related to clients of ours that send us emails through a barracuda gateway.  One of my clients got Barracuda on the phone and they confirmed something on our side was not responding properly and causing their side to issue a RSET (Reset) and close the connection. The fix was to whitelist the inbound domain.  Didn't love that, but it got the job at the time.  

Thanks TIm.  I've had similar issues with FTP and TLS. I am working with the IT from the company sending - and they think it's on our end.  I hate whitelisting IPs...but that might be my answer.  I'm waiting to hear back from them now.

Hi

We have the same problem on 7242. I was able to reduce the occurrence of the problem by disabling the greylisting. 

I did open a ticket, provided dotnettrace and wireshark captures but we did not find a solution over the time of 80 days. As the developers where busy with the new mapi version and had not really time to look at this problem (and second one) we closed the tickets hoped to reopen them if the new version still would have the same problem. I did not expect that the new version will take such a long time. 

I later found some other posts here in the forum that the greylisting.dat is written every 15 seconds to the disk. And as ours was about 100 MB big this could be the issue. A beta version of smartermail did increase the time between writes to 90 seconds. But we don't use betas, so I can't tell if this would fix it in our case. 

Best Regards,

Reto