Undelivered Mail Returned to Sender

Hello to all,

I have an server with I-MSCP installed(Postfix + Dovecot) and in the past couple of days for one of my domains I receive a couple of bounced emails as Undelivered Mail Returned to Sender(random from, reply to, return-path, etc.. set as inexistent-user@mydomain.com) emails that are bounced from Gmail, or different servers(sent to strange email addresses).
Checking the headers or the email source doesn't make more sense, a couple of emails bounced from servers in Germany, Gmail, Yahoo.

The question is how you guys protect against spoofing, the restrictions I've set into Postfix doesn't seem to be working against this kind of useless attack.

Quote from Bulli

Is there a php script which send mails from your server? how many mails are in mailqueue? Check your logs (apache access logs).

The emails are not sent from my server, they are fake and have return address or from field set as an inexistent-address@mydomain.com, which makes that the bounced emails to return to my domain(I have an catch all set up).

The thing is that this server uses DDNS so I can't use SPF(or at least is not working). SPF is set with H.E.

Quote from Bulli

Are you sure? Have you check the logfiles?

Thank you for the responses, yes I've checked logs, no sign of sending emails from my server. From messages headers I can see that the mailer is PHP Mailer(so it must be an some sort of automated script) also Message-ID containing different .ru/.de domains. So basically I think that this is an bot attack.
All I did was to remove the catch all, and this helped me not to receive in an inbox the trash emails, and hopping that in the future the rejected emails will make attackers to quit.