Email address policy customization, all letters or numbers up to @ sign

Hi there

I am trying to create a script that will check whatever is the primary email for a user

lets say david23@test.corp copy whats before the @ sign and assign a new policy

for example david23@testing.com.

the long story:

I have a lot of users that aren't using the default email policy so they have custom email address that I have no clue what they are(thousands of users like that) so if I modify the default email policy that's based on firstname.lastname@test.corp and just add firstname.lastname@testing.com it wont apply for them, so I have to find away to add the relevant email for these.

more of powershell thing then exchange.

Thanks a lot for any advice