root=username@symantric.com
mailhub=mailserver.symantric.com:25
rewriteDomain=symantric.com
hostname=gtmpc
FromLineOverride=YES
AuthUser=username
AuthPass=password
UseSTARTTLS=YES
UseTLS=YES

root:username@symantric.com:mailserver.symantric.com:25
mainuser:username@symantric.com:mailserver.symantric.com:25

echo "Test message from Linux server using ssmtp" | sudo ssmtp -vvv username@symantric.com

[<-] 220 mailserver.symantric.com ESMTP Postfix
[->] EHLO gtmpc
[<-] 250 DSN
[->] STARTTLS
[<-] 220 2.0.0 Ready to start TLS
[->] EHLO gtmpc
[<-] 250 DSN
[->] AUTH LOGIN
[<-] 334 VXNlcm5hbWU6
[->] Z2F5YXRyaS5wYXRlbA==
[<-] 334 UGFzc3dvcmQ6
[<-] 535 5.7.8 Error: authentication failed: authentication failure
ssmtp: Authorization failed (535 5.7.8 Error: authentication failed: authentication failure)