How to stop "could not be delivered" emails?

[shibby2]

Hey everyone, I'm running a 3.8.2 VB forum and for years I've been getting like 20-30 "could not be delivered" emails in my account every day. I have a couple other VB forums and they never get them, but this one gets them constantly.

The email always goes something like this:


Title: Undeliverable Mail Returned to Sender
Error
ERROR: Your message could not be delivered.
Technical error details can be viewed HERE.
The text of the returned message can be viewed HERE.
text/plain Notification (1 KB)
This is the mail system at host gateway30.websitewelcome.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

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

<somerandomemail@126.com>: host 126mx02.mxmail.netease.com[220.181.15.152] said:
550 User not found: somerandomemail@126.com (in reply to RCPT TO command)



I'm sure a lot of them are from spammers, but a lot seem to be from normal, regular user emails. I really don't know a lot about email. I'm assuming this is because the email address doesn't exist anymore? Either way, is there a way to disable/fix this? I'm getting way too many emails, and it only happens on this one forum.

Thanks for any help you can give, I appreciate it!