User change its alias name under Gmail

Dear Experts,

I am managing Gmail accounts for our company, as you know under gmail to create email address, first name, last name and email address required to fill.  one of our users that has email at our domain sent an email with his alias (first name, last name) section changed to something else, which is our company name.

can you tell me how this was done, is it a real security threat or any one can do that?

thanks