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My mail server is set up so that every email to the domain comes to me. One can email whatever_they_want@mydomain.com, and it will forward to me.

I often get spam to strange address. Let's say, Bob_White@mydomain.com - just random names. I get a lot of these, always to different names. On any other mail server, the invalid address would just bounce back to the spammer. So why am I getting spam to these addresses?

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    Note that generally an invalid address should not send a bounce to the spammer, but fail in the same connection, since otherwise it will be sending the bounces to innocent parties whose email was spoofed! – Ángel Sep 3 '17 at 21:57
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I can see at least two reasons.

The first is that spammers know that a number a people like you turn on catchall E-mail. So they are trying their luck.

Second, in addition to using know E-mail addresses, spammers will also fetch lists of domain names and try more or less random combinations. They will of course try info@domain.tld, sales@domain.tld, webmaster@domain.tld, etc but may also use common name/surname combinations. It's some sort of brute-force spamming.

The aim is to reach a maximum number of recipients.

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Few Best Practices to configure on your Email security gateway ti avoid your situation:

1 - Drop the mails with the unknown user (email) - you may sync the Email Security Gateway with the Directory Service or your mail server.

2 - Do not send bounce messages - this would allow spammers to validate the legitimate yours on your mail server. And If spammers sending spoofed email addresses the real users will receive unwanted return messages

3 - Configure your Email Security gateway for threat intelligence feeds where the blacklisted IPs/Domains could be blocked without spamming you.

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