Is it possible to use Guest Accounts in the Target for the email rewrite (cloud to cloud domain rewrite scenario)

We are trying to configure ERS to use mailenabled Guest Accounts in the Target (as the source tenant and target tenant have been in place for some time and as such there are a large number of Guests that have been invited into the Target from the source. Discovery finds the Guest accounts (seen in the logs) but will not display any Target guest accounts in the matching Domain Rewrite > Users + mailboxes window.

We are matching from Source email > Target Extattribute14 (used DirSync to add ExtAtt14 as s.WindowsEmailAddress).

Is matching on Guest Accounts even possible in On-Demand ??

Thanks

Darren