During a journal migration, an envelope is created for each message. Sender information (from the P1) is needed to create the envelope. When the sender is missing, creation of the message envelope fails.
When this happens, the SmtpResolve function is used to get the PrimarySmtpAddress for the sender from the Archive Shuttle database.
If that information is not available, then the address in the SourceOne Module setting (shown below) is used to create the envelope.