I am using a e-mail trigger event to pick up an e-mail from a user. This user i

As long as your capturing the to / from / subject etc you can then use that to lookup the user against a database table which is my preference. I am not sure if you can lookup user properties by email address using any of the Appian functions, someone else might know if that is possible.

I use this to lookup the to part of the email to assign the email to a certain team based on a process name / group id configuration.

Assuming all your users are in a group (such as an All Users group or a smaller group), you could run a statement in a script task like wherecontains(ri!emailFromTrigger,apply(user(_,"email"),getdistinctusers(topeople(cons!AllUsersGroup))).

This would return the index, so then a statement like index(getdistinctusers(topeople(cons!AllUsersGroup)),IndexFromAbove) would return the user

Great! Thanks both of you!