Question : "stat=deferred" in sendmail log and messages not received

Most of my emails get through to the recipients, but several intended recipients recently have told me they have not received my emails.  When I check the sendmail log for each of these, I see:
"stat=deferred" (usually followed by "connection reset by...")

Looking further at the log, there are a variety of "stat=deferred" messages with various messages like "operation timed out", "greylisting in action", "please try again later", etc.  (I can't tell whether most of these were received or not).

Do I have a problem at my end, or is it an issue with the recipients?  

 

Answer : "stat=deferred" in sendmail log and messages not received

> Based on the above info, would you say that the problem is at the receiving end, or is there any reason to suggest something needs fixing at my end?

I don't know how your sendmail is configured. Hardly it configured to keep message for 3 weeks in a queue. It may be corrupted mail in a queue, so it can't be send and it's save to remove it manually.

One more possible reason is enabled path MTU discovery. I told you how to disable it. Just disable and try to resend.