Hey perfectweb,
Sounds like you're mentioning a contact within your iMessage, but don't want it to do so. We'd be happy to help out.
The behavior you've described is expected behavior. You can mention a contact when using iMessage with one of your contacts by either typing their contact name and clicking on the name or by using @[their name].
To help bring your message to the attention of a recipient, include @ before their name, like @John. Or click their name after you type it. Their name is then highlighted in the message, and they will receive a notification letting them know that they have been mentioned. This feature is available in macOS Big Sur or later.
This information and more is outlined here: Mention a person in a message on Mac - Apple Support
If you don't want this to occur, don't click on the name after typing it and it should appear as expected (not in blue) or in the case of the email you're attempting to type, type [first name]@[first name], tap Space, then type [last name] and then backspace the space between first and last name. That worked on our end to prevent it from turning blue. If that doesn't work though, you could also adjust the name in your Contacts app so that it doesn't match the email exactly and that would prevent this from occurring in the future with no workaround needed.
While it's not common for people to have their exact full contact name as the ending of their email address, when you do occasionally run across this, the above steps should help you out with avoiding the conflict it creates.
Take care!
