When an email arrives in the Email Cloud, the first thing Match My Email does is consider ignore rules. After passing that step, email addresses in the header are used to determine if there is a match or not to Salesforce records at that point in time. The message then has a status with match information or of No Match. While No Match emails are automatically reprocessed every 20-minute cycle, messages that already have been matched are considered “done”.
Reprocessing can be thought of as having the Match My Email system re-analyze the email addresses in the header as if the message was arriving in the Email Cloud for the first time. Therefore, if a change is made to ignore rules and the User wants that change to affect emails in the Email Cloud, matched or No Match, the User would need to manually reprocess an email message. This would have the system look again at Ignore rules and apply any changes to selected email messages.
If an email address is added in Salesforce, perhaps for a new Contact, and email was sent to or from that individual, as long as the message is a No Match status, the automatic reprocessing every 20 minutes will find and upload the email. However, if an email was already matched into Salesforce to perhaps other Contacts at the same Account, but the new Contact wasn’t in Salesforce at the moment of matching, the message would need to be manually reprocessed to attach it to the new record.