There are a million reasons it could be detected as spam. It really depends on what type of spam filter you have. This isn't an issue with your program, it's an issue with your spam filter.
This is always an ongoing problem with my forum as well. Users may never receive an activation email.
Only thing I can recommend is make sure they know they were supposed to receive an email and to check their junk list. If they didn't get one you may have to send it manually which always seems to get through.
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