What information to include is the 'info' input for HKDF?
No, the info is not a salt. The input key material for a KBKDF (key based key derivation function) should already be pseudo-random, and should therefore contain enough entropy to not need a salt.

If the user already has a unique 16+ pseudo-random bytes key then there is little reason to use the email address as part of the info. The email address could be used to make the derived key specific to the user, but that is already the case (in your example).

The main reason for a KBKDF is to create more keys or longer keys. You would need to use the algorithm name as part of the info when the algorithms would otherwise use the same key material.

The info is the key derivation data. You use identical input key materials to create multiple derived keys. The info makes the distinction between the keys. The KBKDF makes sure that the keys are indistinguishable from pseudo random numbers (and thus independent of each other).

Example uses: create a encryption and mac key from the same input key material (info is ASCII encoding of "ENC" and "MAC" or a counter value 1 or 2).

Another example: encrypt files with the same key without using an IV: input of the info part is the UTF-8 encoding of the file name.