We have a secondary email address field that our reps use if they can't get through to a lead or the primary email address bounces, and if it works they can replace the primary email address with the secondary email address or another new email address.
We also use the secondary email address in a couple of other ways - both manual, but it would be great if Marketo could help automate this.
Frequently, our reps reply to emails they receive from known leads, but the email address the lead used in sending the email is different from the primary email address on their lead record, which results in a MktUnknown lead record being created. If the email address associated with that unknown lead is in the secondary email address field on the known lead record, we can find it via search and merge the unknown and known lead records together.
When a known lead fills out a form, but they use a different email address from the primary email address on their lead record, this results in a new lead record being created for the same person. When this happens, we receive a duplicate name alert and If the email address associated with that new lead is in the secondary email address field on the known lead record, we can find it via search and merge the two lead records together.
Marketo should provide the ability to match a secondary email address to existing records, rather than creating a new record.