NOTE : Personal identification has been replaced by bogus values.


Hello Forum Members,

I am facing issue of SMTP authenticatiion failure while trying to relay mails thru my ISP smtp server (99.99.18.19)

My local zimbra server was communication very well with ISP server acting as smart host using SMTP authentication.

One fine day the relay started to fail. As per my knowledge, no change whatsoever was made to the Zimbra or to the CentOS.

To set the authentication relay, i had successfully followed the below article :
Outgoing SMTP Authentication - Zimbra :: Wiki


When the issue started, i reapplied all the commands mentioned in the above article, but was of no help.

User name and password used for SMTP authentication had been verified by logging into the mailbox using Webmail.

Any help in this regards would be highly appreciated.

The actual error message , while trying to send mails out is as below :

<username@hotmail.com>: host 99.99.18.19 [99.99.18.19] said: 550
Authentication is required for relay (in reply to RCPT TO command)


Thanks in advance.
Rehan