[otrs] Invalid address but it is correct

I am trying to compose a reply to a customer but OTRS won't let me send the mail because it says that the email address is invalid. The address seems correct. It looks like some.name@abc.com but OTRS reports "* invalid some.name@abc.com!" Why is this? -- Wes Plate Automatic Duck, Inc. http://www.automaticduck.com