I did actually try signing up with a fake email
not@not.com and replicated getting a undelivered notification though. And it opened an account. But obviously there was nowhere to send an activation email to. So maybe I need an email filter?
not.com is a valid domain that belongs to "Next Online Technologies":
It also has a valid DNS entry for the mx:
$ dig -t mx not.com
; <<>> DiG 9.10.6 <<>> -t mx not.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 55124
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;not.com. IN MX
;; ANSWER SECTION:
not.com. 300 IN MX 75 isaac.mx.cloudflare.net.
not.com. 300 IN MX 94 linda.mx.cloudflare.net.
not.com. 300 IN MX 30 amir.mx.cloudflare.net.
So the domain exists, the mx entry exists but the mailbox for the localpart of the address does not exist, thus you get a bounce. Completely different story from what you wrote in your last post.
Anyone can use any mailaddress during registration and as long as there's an mx entry for the domain the server will try to deliver the registrasion mail (as it does and can not know if the mailbox/mailaddress exists). A "mailfilter" whatever that may be cannot circumvent that - how should it do that?
First thing you should find out is wether the mails you get are caused by registration attemps or by the contact form. Should not be too difficult. If they come in masses it is probably automated and then the according spaminator variant could probably help.