[tag:email-rules] has 1 follower and 61 uses:

>Actions performed automatically on incoming or outgoing messages, based on conditions that you specify.

[tag:email-filter] has 0 followers and 48 uses:

>Email filters allow custom actions to be performed when a message satisfies the filter/rule requirements is received. An example of a common email filter is spam protection, which automatically moves all incoming messages into a Spam folder (action), when a given message is determined to be "spam" (rule).

Based on their definitions, these tags are clearly synonyms. I'd like to merge them. [tag:email-rules] has the slight edge in usage, so I suppose [tag:email-filter] should be made a synonym of that.