Postfix deferred email – How to expire so bounce-back email gets sent back

This happens when domain.com has an MX record (or, in the absence of an MX, an A record) that points to your Postfix server, but your Postfix server is not configured to accept mail for that domain. It's commonly seen in two situations:

1. You've acquired a new domain that you want to use for email, but you just forgot to add it to Postfix. There are a couple ways to do it. In my case, I'm using the virtual table to map addresses to real local users, so I list my domains in the virtual_alias_domains directive in main.cf.
2. The domain has its MX record set to a host that resolves to 127.0.0.1. Some malware will use this trick so they can set their return address to a valid domain, which most anti-spam measures will check for. However, the MX effectively prevents any delivery to that address, so the sender doesn't have to deal with replies or bounces.

Yep, Windows Live can be a pain. Check:

• You're not on a blacklist
• Your server has MX and reverse DNS records
• You have SPF DNS records (many servers reject mail without a valid SPF, GMail for example, here's an explanation and a wizard)
• Your mailserver's HELO response matches your hostname
• Your mailserver is not an open relay
• Your DNS records' TTL is not too low - 86400 (24 hours) is recommended (some spammers set their TTL very low to regularly update forged DNS records)