Sendemail Error - Authentication... failed

SCO Unix OpenServer v6.
We use the script below for sending mass emails with attachments.

sendemail -f $From_user -o message-file=/u/fg4/data/EmailDefaultBody.html -u $su
bj -s $MAIL_SRVR -xu $MAIL_USER -xp $MAIL_PSWD -a $emlname.pdf -t $MAILTO

MAIL_SVR=SMPTOUT.SECURESERVER.NET

Recently, we have been getting error message on SOME of the email documents:

EMAIL file name : 01003000A
Oct 09 11:25:33 mail sendemail[14958]: ERROR => ERROR => SMPTP-AUTH: Authentication to SMTPOUT.SECURESERVER.NET:25 failed.

Most documents go out with NO error. I have found no Spamhaus or blacklist entries in the mail logs.

Any leads on how we can resolve this is greatly appreciated.

Thanks,
jet47