Unable to send an email: 535 5.7.8 Error: authentication failed: authentication failure

Unable to send an email: 535 5.7.8 Error: authentication failed: authentication failure:-

we can be found this exception in /var/log/maillog :

Dec  4 06:06:34 servername postfix/qmgr[6334]: DA7FB101118C: from=, size=827, nrcpt=1 (queue active)

Dec  3 04:04:22 servername postfix/error[25863]: DA7FB101118C: to=, relay=none, delay=0.1, delays=0.1/0/0/0, dsn=4.7.8, status=deferred (delivery temporarily suspended: SASL authentication failed; server someexternalserver.com [203.0.113.2] said: 535 5.7.8 Error: authentication failed: authentication failure)

relayhost is configured in /etc/postfix/main.cf :

# cat /etc/postfix/main.cf | grep  "relayhost ="

relayhost = someexternalserver.com

Cause

Issues on someexternalserver.com side.

Resolution

Contact owner of someexternalserver.com to check the issue.

2.1. relayhost can be disabled by commenting it in /etc/postfix/main.cf

    # cat /etc/postfix/main.cf | grep  "relayhost ="

    #relayhost = someexternalserver.com

2.2. Restart postfix:

    # service postfix restart