The address you're noting ("bounces+12345...") is the Return-Path address, not the FROM address. We overwrite the Return-Path address in order to handle bounce management for you. However, mailbox providers should be displaying the DATA FROM address, not that Return-Path address.
When seeing this issue there can be many reasons for it. Most often this can happen when sending via SMTP and not supplying a DATA FROM address.
If you cannot resolve this on your side, you can tell us to not overwrite the address, but we don't recommend this as it will significantly hamper our ability to manage your delayed bounces and spam reports. Essentially, we will be unable to log delayed bounces or spam reports to the account suppression lists, and will subsequently not suppress future attempts to send to these recipients.