Ultimately, the lessons I took away from my own experience are these:
If you haven’t already created an SPF record for your domain, do so.
If your mail server allows its users to forward mail off-site, then implement SRS remailing on it ASAP.
Note to author: it's better to put the conclusion at the beginning instead of burying it 1000 words down. What you learned is more important than how you learned it, even though the latter ultimately validates the former.
TL;dr
Ultimately, the lessons I took away from my own experience are these:
If you haven’t already created an SPF record for your domain, do so. If your mail server allows its users to forward mail off-site, then implement SRS remailing on it ASAP.
Note to author: it's better to put the conclusion at the beginning instead of burying it 1000 words down. What you learned is more important than how you learned it, even though the latter ultimately validates the former.