Short answer: yes Long answer: it depends. On the settings on all the parties involved. Mail bounceds after the retry limits and timeouts are exhausted. Your

Short answer: yes Long answer: it depends. On the settings on all the parties involved. Mail bounceds after the retry limits and timeouts are exhausted. Your provider tries 5 times over 24 hours? Then that's how long fastmail has to be unreachable by your provider. Maybe my provider reaches them via their secondary routes and thus there's zero chance of bouncing. Mail goes through right away. At least to their server. But maybe the recipient reached their servers via the attacked primary and thus can't see/fetch that email.