bounced email

one of my user reported that her client/customer always received bounced emails when sending to her.

Mail delivery system

error message

I'm sorry to inform you that your message could not be delivered to one or more recipents.

for further assistance, please send mail to postmaster.

if you do so, please include this problem report. you can delete your own text from the attached returned message.

the mail system

myuseremail@mycompany.com.sg : host mycompany.com.sg.protection.outlook.com said :452

4.5.3 too many recipents (AS780090)  [AMXgf.protection.gbl] (in reply to RCPT TO command).

the client can send to other 2 users in my company. only this particular cant received the emails.