Could not send emails

After updating to 18.1 on our on-premise installation email alerts have stopped,  i can see the alerts under system alerts but those alerts are not being delivered this is also impacting new users not getting email notification. I have checked custom.properties and password.properties for any changes but all settings are as they were before. Under mail-listener  i see this "[ServerService Thread Pool -- 65] INFO  com.appiancorp.ra.emailpoller.EmailPollerRa - Email listener not configured (mailServer); will not be activated." 

When i looked into unattended-request-poller i found the below error message,

Caused by: javax.mail.SendFailedException: Invalid Addresses;
  nested exception is:
    com.sun.mail.smtp.SMTPAddressFailedException: 554 5.7.1 <user@abc.com>: Sender address rejected: Access denied

    at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:1873)
    at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1120)
    at com.appiancorp.ap2.mail.Mail.sendMessage(Mail.java:329)
    at com.appiancorp.ap2.mail.Mail.sendMessage(Mail.java:133)
    at com.appiancorp.ap2.mail.Mail.sendMailWithAttachment(Mail.java:200)
    at com.appiancorp.ap2.mail.Mail.sendMail(Mail.java:162)
    at com.appiancorp.process.engine.AbstractMailRequest.sendEmail(AbstractMailRequest.java:272)
    ... 66 more
Caused by: com.sun.mail.smtp.SMTPAddressFailedException: 554 5.7.1 <user@abc.com>: Sender address rejected: Access denied

The credentials are correct i tested by logging in the mailbox and sending test email.