On my Incoming Gateway, I enabled Recipient Verification.   The incoming gateway is SmarterMail Free.   The machine being queried is another vendor's product, functioning as a second-pass filter for incoming email.

After running this way for several weeks, I discovered that on a random basis, some messages were being incorrectly rejected as "no such user here".   In at least one case, this caused the user to be dropped from an important mailing list.   Preventing a re-occurrence became critical, so I disabled the feature and am afraid to turn it back on.

I have asked SmarterMail to implement a way to activate Recipient Verification in test mode (ask the question, record the results in the log file, but do not block the message.)    This would allow me to work with both ends of devices and their vendors, in hopes of finding and fixing the cause of the problem.

This is an instance of a larger principle:   For any potentially disruptive rule change, the system design should allow the rule to be activated for data collection purposes only, with logging but without enforcement.

As always, SmarterMail encourages me to get supporting votes from the communtiy.