userId match type restrictions (no replies)

Hi,
I am working on creating segments for users on an intranet and have used the users' emails set as userId. I want to create a segment that matches the domain end of that email/userId. for example:

With userIds like this:
bob@department1.com
alice@department2.com

I want to have segments like this:
"Department 1" segment: userId =@ department1.com
"Department 2" segment: userId =@ department2.com

but I cannot, I get the error:
<error message="The specified segment is invalid: Invalid Segment match type: try using 'userId' segment with one of the following match types: ==, ::NOT_NULL, ::NULL, !=."/>

1) Why does this match type restriction exist? ( Also, why isn't it documented anywhere, so that we can know ahead of time whether the solutions we try will work?)

2) What are some possible workarounds? Do I need to set up a custom variable that contains only the domain part that interests me?

Thanks!