ORA-29279 SMTP : 550 5.1.1

Hello

Have this error when using the package UTL_SMTP for email adress containing number.

     ORA-29279   SMTP : 550 5.1.1

Sending mail work fine for email adress like "xxxxx.yyyyy@domain.com" where xxxx or yyyy are all characters. but when adress mail is like "xxxxx1@domain.com" or "xxxxxx2@domain.com" or any number sending mail  dosn't work. All the email adress are valid and sending mail from oulook work fine.

Have any idea?

Regards