Hi,
As Robert price mentioned, you can use the email regex from community commons. Here is the screenshot of how we use email regex in Mendix.
You can replace the last part after @ with your constant.
Hope it helps.
If you are looking to validate an email address, try using the EmailAddressRegex in the Community Commons module.
https://docs.mendix.com/appstore/modules/community-commons-function-library/#48-regexes