I'm having a little trouble narrowing down the cause of an intermittent mail flow issue for a client.
A few times a week, an external user receives a bounce-back from my clients mail server, here is the important information from the bounce-back message:
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<mail.remote.com #5.5.0 smtp;550-(mail.remote.com) [98.X.X.X]:1939 is currently not permitted>
Obviously the client's actual domain is not "remote.com", this is merely a placeholder. Also, while the mail may get rejected randomly, waiting a few minutes and re-sending appears to work for the time being.
Pertinent information about my client:
1. My client's internet connection is currently under fairly heavy load, which does increase their MX server's response time. An upgrade is already being looked at for the connection, we're waiting on the ISP.
2. I have received no other complaints from my client regarding external e-mails that are getting rejected or not coming through.
3. My client uses MXLogic, however, I have verified via a mail audit and a phone call with their support that these e-mails are not getting rejected by them. In fact just to be safe, I've added a *@domain wildcard and a specific entry for the user (redundant, I know).
Pertinent information about the remote sender:
1. I setup a PRTG probe to monitor SMTP traffic on their MX server, it can spike up to 1800ms response time at certain times throughout the day (usually 0830-0900) and is averaging out at 300ms. However, I was not able to match up any bounce-backs to any spikes on their mail server.
2. I called the remote sender's IT company and they did not report any issues.
My Current Theory:
Due to the stress of my client's connection and the stress of the remote sender's server, perhaps this is just too much delay and is causing a timeout.
If you need any more information from me, please let me know, thanks!