how can i configure email on my SAP

I want to send emails form my SAP system. I configured SMTP server from SICF and then in SCOT. Emails were going fine till tomorrow but they are not going today. it giving error like

Message cannot be transferred to node SMTP due to connection error (final)

I dont know what is the error and why it was working yesterday and not today..

can anybody help me in this??

Regards

Arpit