Right now (Mercury 4.62) the only way for the VERP processor to suspend/delete a subscriber whose e-mail is bouncing is when the corresponding total error count exceeds a certain fixed threshold level either set to "N messages per week", "N messages per month" or "N messages in total".
Considering that the delivery errors are often transient, ie they usually get fixed in a relatively short period of time, I think it would be useful to have fourth error count mode (which I call "max-errors-in-a-row"), in which a subscriber would be suspended/deleted only if the delivery failed for "more than N messages consecutively". This would be quite trivial to implement, because the error count just needs to be to reset to 0 each time a message is successfully delivered.
In my opinion this additional mode could prove more effective than others in certain situations, such as the case a low-traffic distribution list in which each subscriber receives just a few message per month in the average (in this situation the other modes seem to be too much draconian).
Just my 2 Euro-cents :-)
Thanks for the attention,