JBossAS deploys a mail service as part of its standard configuration. The mail service takes the form of a javax.mail.Session, and is bound to JNDI under java:/Mail. Applications can make use of this mail service in order to send mail via SMTP.
My guess is that your seam app is trying to send mail by one method, and your servlet app by another method. Judging by the error message you're getting the servlet app is doing it using the JBossAS mail service, and the seam app is trying to do it all itself.
In order to configure the JBoss mail service, you need to edit the deploy/mail-service.xml file under the server directory.
As for the Seam app, you need to find out how it's doing mail handling, and configure it seperately to the JBossAS mail service. Better yet, change it to use the JBossAS mail service.