From some addresses it's impossible to post into a smartlist

Hello

I've been running a couple od mailing lists using smartlist for years. I'm using
procmail version 3.22-5 (standard Redhat 7.3) + smartlist 3.15. Recently I've
ran into a very strange problems which I think that it might be a bug or
inconsistency in smartlist. From some email addresses it's impossible to post a
message into a mailing list. The message gets bounced to the list maintainer.
All I get in log file is:

Subject:
  Folder:  formail -R"From X-From_:" -iDate: -iReturn-Receipt-To: -iRe     1440

In the bounced message header there is:

X-Diagnostic: Already on the subscriber list
X-Diagnostic:   4 [email protected]              32720 [email protected]

The email is already on subscribe list of course, but other emails that are
there too work fine. This mostly happens with emails containing . character in a
name of a person. But not all of them, some work fine. Seems like some
combination of mail header problems.

Any suggestions or ideas what to do, or where the problem might be?

Thanks in advance for help.

Vladia