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Old 25 Oct 2018, 03:45 AM   #5
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Join Date: Apr 2008
Posts: 362
Sadly, that's not even really a guarantee anymore either.... Do a reverse DNS lookup on whatever IP address you have at home, and you'll very likely find that it resolves to something —*usually just an amalgam of your IP address as a subdomain of your ISP. There are blacklists (RBLs) out there that include ranges of dynamic "residential" IP addresses for this reason (Sorbs' DUL and Spamhaus' PBL come to mind), but very few mail providers actually use them, and FastMail definitely doesn't.

Ultimately, however, "noreverse" should be very uncommon these days, as it's usually the result of a misconfiguration, or somebody using some very small and obscure ISP who doesn't really know what they're doing. It's definitely not a bad thing to use it as a factor in spam scoring (and I'm fairly certain FastMail already does this), but it would be a mistake to assume that a message should be discarded simply because a reverse entry can't be found to match the sending IP address.

Keep in mind also that this is a dynamic lookup that occurs when the message is actually received by FastMail. I had one client a few years ago who ran their own mail server and had configured their firm-wide spam filters this way. They ended up losing several hours worth of ALL legitimate mail simply because of a DNS lookup problem (it looked to their server like nobody had a valid PTR record, so every single message that came in was discarded as spam). While that's less likely to happen to Fastmail, there are a lot of pieces to DNS that can cause problems out there on the internet at large, and just because a server can't find a reverse DNS entry for a given IP address, it's no guarantee that one doesn't actually exist.
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