Where can I see the actual messages of the bounce backs I got?

If you wish to review the actual bounce messages, you can either:

1. Uncheck the Delete after processing option for your arpReach mailbox (Setup / System Settings / System / Incoming email), access your #V2 system mailbox account (e.g., via webmail) once you get bounces, then review the messages there

2. Set a forwarder so that a copy of each message received by your system mailbox will be sent to another mailbox (this way you’ll prevent duplicate processing and the mailbox from being full)

Note that if you will be choosing #1, we strongly recommend that you re-enable the Delete after processing option once you have the data you need, as keeping it disabled will result to duplicate/multiple processing of the emails received by your #V2 mailbox.

If you need to review your bounces for a long or indefinite period of time, going for #2 is the best option.