Spam with fake from field

[marko1103]

Hello,

We have few shared hosting servers with Plesk installed.

Lately we had many problems with users e-mail credentials being leaked out to spammers and than they send a lot of spam through our servers.

I`ve added hash:/var/spool/postfix/plesk/blacklists to smtpd_sender_restrictions in main.cf of Postfix configuration but this only helps if from field matches blacklisted address.
But if the domain is correct and from field faked e-mail is still sent.

Anyone experienced same problems?
What is the best solution to stop spammers from sending e-mails if their from field is fake?

basically I have to configure postfix to check if mailbox exists on mail server but we have few thousand e-mail addresses so I cannot manually add such file.
It there any simple solution for this problem?

Thanks

Kind regards,
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Marko