I went through the journey of configuring Postfix for a small side project I was working on. The idea was to accept emails sent to a unique, per-user address a

I went through the journey of configuring Postfix for a small side project I was working on. The idea was to accept emails sent to a unique, per-user address and forward all attachments to the user’s cloud storage account. I looked into using mail services like SES and Mailgun, but given the average expected email size and the fact that I would primarily receiving mail, the additional cost didn’t make sense. Along the way, I ended up learning quite a bit about Postfix filters, virtual domains, SPF, and DKIM. I also learned how quickly spammers can detect open relays. Apparently, while copy-pasting a config from somewhere, I had a slightly incorrect 172.x subnet listed as part of “mynetworks”. As a result, a spammer with a machine belonging to this subnet was able to use my server as an open relay. Long story short, it took me quite a while to root cause the issue and get my domain off the blacklists!