Dynamic IP: If you're in a dynamic IP block (say, from your cable/DSL provider) you're going to fall afoul of MANY spam-blocking tools, as that's where a ton of bot-infested PCs reside.
DNS/Reverse DNS lookup: In an SMTP conversation, your host identifies itself by name - HELO foo.bar.com for example. If foo.bar.com doesn't resolve to the IP address the connection is coming from (or the IP address doesn't resolve to the name in a reverse lookup, I forget which is checked, or if both are), then that's another red flag for anti-spam systems.
MX record: Each domain has one or more MX (Mail eXchanger) records in DNS - if your machine is sending mail from foo.com, but it's not listed as an MX for foo.com, then there's a possibility that it's a spam engine. |
