Since early versions (2.3), YAM has a 'bounce' (remail) option which allows to send a mail to an alternative recipient as it would have been sent to that recipient originally. This functionality is also often called a 'remail' operation. What YAM does is, to take the mail as you have received it and directly remail it to an alternative recipient. The new recipient will receive that mail with more or less not noticing that it was send to an another person previously.

In fact, 'more or less' in this term means, that YAM will add some special header entries (e.g. 'Resent-From:' header) to signal that the mail in question was not really comming from the original author directly, but 'resent' instead. This is to conform to RFCs and to give the new recipient at least a minor hint that the mail in question may have been altered on the way.

So if you once received a mail on accident which normally should have gone to someone else you can use the bounce functionality to do that. Another use case could be to forward the mail as is to another account of your own.

However, this functionality is not meant to be used to bounce the mail back to the original author. This includes that e.g. if you received a SPAM message you want to bounce it back to the original author (the spammer) to give him the impression that something went wrong and that the user didn't exist. That's not only producing unnecessary email traffic but also isn't the scope of the bounce functionality in YAM. In addition, since 95% of the junk mail have bogus sender addresses this would result in error messages or that you have sent SPAM yourself to someone innocent. Spammers themself don't care about whether the mail makes it or not (hence the bogus address). All they are after is the small percent that fall for whatever they are spamming for. If the address is simply bad then it is going to bounce back or more commonly it is someone else's address that is going to get bombarded with bounced messages.