I am running Outlook Express 6 and out of the blue it won't send e-mail but it receives e-mail. [I get the error : The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was XXXXXXXXX'. Subject 'Test', Account: 'mailhost.spf.ameritech.net', Server: 'mailhost.spf.ameritech.net', Protocol: SMTP, Server Response: '550 [PERMFAIL] hotmail.com requires valid sender domain', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79.] I get this error no mater who I send an e-mail too (not just hotmail). I have the same account set up on a different computer with all the same account info., servers, ports, passwords, authentication, etc. and I can send e-mail from that machine. So the problem must reside with this computer. I have done a system restore to a date previous to my last successful e-mail sent and this changed nothing. I have disabled my windows firewall and I cannot send as well. Any suggestions? Baffled.