Remember that starting in May 2023, bounce-backs due to invalid address are automatically deleted, but we also delete addresses where the mailbox was full or blocked. (We do ignore "out of office" replies because those aren't considered bounce-backs, just responses to the initial email.) 

Members sometimes provide an email box that they don't monitor at all. But that is a risk for the entire network, because if we keep sending mail to those mailboxes we are at risk of being blocked by spam monitoring services.  So the member must provide an address that they actually maintain.

Also refer to the related links below for other reasons that might be affecting these emails being delivered.