[email protected]
mailhub=smtp.office365.com:587
rewriteDomain=skulocity.com
AuthUser=customerservice
AuthPass=mypassword
FromLineOverride=YES
UseTLS=YES
UseSTARTTLS=Yes

#########################

root@PBXSERVER:/etc/ssmtp# echo "Test message from Linux server using ssmtp" | sudo ssmtp -vvv [email protected]
[<-] 220 BN6PR03CA0021.outlook.office365.com Microsoft ESMTP MAIL Service ready at Mon, 7 Jan 2019 04:13:27 +0000
[->] EHLO PBXSERVER
[<-] 250 SMTPUTF8
[->] STARTTLS
[<-] 220 2.0.0 SMTP server ready
[->] EHLO PBXSERVER
[<-] 250 SMTPUTF8
[->] AUTH LOGIN
[<-] 334 VXNlcm5hbWU6
[->] Y3VzdG9tZXJzZXJ2aWNl
[<-] 334 UGFzc3dvcmQ6
[<-] 535 5.7.3 Authentication unsuccessful [BN6PR03CA0021.namprd03.prod.outlook.com]
ssmtp: Authorization failed (535 5.7.3 Authentication unsuccessful [BN6PR03CA0021.namprd03.prod.outlook.com])