Yahoo mail, along with most other crappy webmail accounts, puts a little tagline at the bottom of all the emails you send. Free advertising for themselves, and a good way to annoy anyone else having a conversation with you – you rapidly start pushing around loads of nested adverts whether you like it or not.
However, this amused me, from the climbing list I’m on – Yahoo! put the following tagline in a poor guy’s email:
All New Yahoo! Mail – Tired of Vi@gr@! come-ons? Let our SpamGuard protect you.
… which then got promptly munched by the anti-spam filter on the mailing list. Nice work, Yahoo!
So if you use Yahoo!, or Hotmail, or any other crappy free email, and every now and then you emails don’t get through, maybe this is the reason.