email-rules has 1 follower and 61 67 uses:

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

email-filter has 0 followers and 48 51 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. email-rules has the slight edge in usage, so I suppose email-filter should be made a synonym of that.