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Here's an incomplete list of more or less related categories:

Category:E-mail(
 
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Category:Authentication methods(
 
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Category:E-mail authentication allows to reduce the number of categories for articles belonging to both categories.

Category:PGP(
 
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That's not exactly true, OpenPGP is also used in contexts unrelated to E-mail like Usenet. Adding only some articles in Category:PGP to Category:E-mail authentication instead of the whole Category:PGP might be better.

The spam categories are as expected messy. Today spam is used in many contexts not more limited to E-mail. Historically the term was only used for Usenet spam. In E-mail the technical terms are Unsolicited Bulk E-mail (UBE) or Unsolicited Commercial E-mail (UCE) - not all UBE is also UCE and vice versa, and some UBE is not necessarily spam.