Improve bounce management: allow viewing of bounce mails and removing from blocklist

[tricoos]

After sending ~2000 mails through AWS I was shocked to see that AWS only showed a 0.6% bounce rate, but listmonk showed a 4% bounce rate. For me this indicates that the bounces were not handled correctly and that some maybe vacation replies were considered as a bounce and caused adding the affected address to a blocklist.

However, there is neither a way to analyze this as all mails are read via POP3 and removed from the server once listmonk has parsed them. There is also no way to see the e-mail and manually fix a false-positive here.

Also, POP3 also deletes all messages that are not even relevant and were not caused by any listmonk campaign. That should never happen. As there is a specific command to delete messages on the POP3 server this should only be issued when the relevant e-mail was the result of a listmonk campaign.

[MaximilianKohler]

Most likely the difference is that listmonk also recorded soft bounces. I have no clue about POP3, but AWS has an option to send you emails for every bounce (see bounce processing here: https://github.com/knadh/listmonk/issues/120#issuecomment-1314110010), and listmonk has a list as well /admin/subscribers/bounces.