In some places (e.g. title of artifacts) the sign @ triggers the creation of an hyper-link even if it does not correspond to anything.

valid.name@valid.email.com is a valid email and should be a mailto link probably
valid.name@somename is not a valid email and should not link to anything

@disciplus_simplex is a valid user and should refer to the user
@marsattack is not a valid user (at the time of writing) and should not link to anything.

NB: patterns which validate emails may be configurable as a site config as some email may be invalid on the internet but valid on intranets (e.g. bob@myserver)