The problem does not seem to be happening for all recipients on a list. It may happen for some partial number of entire list. As a result, we assume that we never sent an email to these people as there were not added to a list.
Logging onto the Constant Contact website, we figured out that even though we get this error, the users are still added to the list AND sent an email.
Does anyone have any idea about what is going on?
We wouldn't be able to see what was going on with out the XML you are sending to our servers at the time of the error. The next time this happens to you, please let us know and include the XML.
We'd be more than happy to look into the issue.