It seems rational to me to assume that if a mailing is set up where a group (or past mailing) is excluded, then any records that share the same email address as an excluded record, should also be excluded.
Indeed, to my mind this is totally aligned with the principle behind the "Dedupe by email" option in CiviMail, that stops multiple copies of an email going to the same address, where multiple records share that address.
However in practice this appears not to be the case, which I found out to my disappointment: a mailing was sent to a group (group A) in error, when it should have gone to a different group (group B). As soon as we spotted the error we sent out an apology message as the mailing was leading to some confusion amongst recipients. However, as we were aware that some contacts were in both group A and group B, we decided to send the apology message to group A, but excluded group B from the mailing, because for those people the mailing was not erroneous. Unbeknown to us at that time there were a handful of additional contact records in group A that shared the email address of records in group B. The result was that although we assumed that no email address in group B would be sent the apology, in fact a small number did get the apology leading to further confusion.
What's the opinion of others on this? I understand that things can get tricky when trying to handle the nuances of scenarios where multiple records share the same email address, but in this situation where we were deliberately excluding a set of addresses, it does seem sensible to ensure that those addresses get removed from the mailing, even where that involves records that share the address.