- They are subscribed to the same list under more than one e-mail
- Their mail server (the computer which accepts mail from us) is
improperly closing the network connection before giving us an
acknowledgement that the message was received. In our logs, our
mail server records that there is a possible duplicate
because without acknowledgement, our mail server has no way of
knowing whether the messages was successfully delivered.
What Should I Do?
Have the subscribers with problems notify their administrator that
this is happening. They may need to contact their vendor for a
fix. This problem is dependent on how their system communicates
with the connecting system so they likely are not experiencing this
problem with every message that is delivered to them. Further,
some connecting systems may make more liberal assumptions about
whether a message was delivered or not.
If the problem takes some time to resolve, you can always suggest
that the subscriber use a free email account (e.g., Yahoo!) to
receive list e-mail.