Caused by: com.sun.mail.smtp.SMTPSendFailedException: 550 5.7.60 SMTP; Client does not have permissions to send as this sender
at com.sun.mail.smtp.SMTPTransport.issueSendCommand(SMTPTransport.java:2202) ~[javax.mail-1.5.2.jar:1.5.2] at com.sun.mail.smtp.SMTPTransport.finishData(SMTPTransport.java:1980) ~[javax.mail-1.5.2.jar:1.5.2] at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1197) ~[javax.mail-1.5.2.jar:1.5.2] at javax.mail.Transport.send0(Transport.java:254) ~[javax.mail-1.5.2.jar:1.5.2] at javax.mail.Transport.send(Transport.java:124) ~[javax.mail-1.5.2.jar:1.5.2] at org.apache.commons.mail.Email.sendMimeMessage(Email.java:1411) ~[commons-email-1.4.jar:1.4]
Seems that the user you used to authenticated against the mail server doesn't have permissions to send emails as the user specified in the mail task. In the mail task are you using the same email (for the from email address) as the address in activiti.config.xml?
Does the mail server use SSL? if so then you might have to set mailServerUseSSL=true. Also can you try setting the mailServerDefaultFrom to an email that is allowed to send emails by the authenticated user.