When an email is sent to a candidate for the first time (for example, a confirmation of receipt), an email address is generated for this candidate. This is displayed to the candidate as the sender address (From:). This is the candidate email address.
It is always: candidate-lettersand/ornumbers@pleasereplacewithyoursystemurl.com
This email address is systemically the anchor point for correspondence with the candidate. A candidate responds to this and the complete communication history then appears visibly for you in the candidate's history.
Note
If it happens to you that emails from the candidate are quarantined by your email server or marked as spam, you can whitelist the address on your email server.
Either with candiate-* This is a bit more restrictive, because then only emails from the candidate are whitelisted.
Or starting with @ so: @pleaseReplaceWithYourSystemurl.com.