A spam trap is a type of fraud management technique used by major Internet Service Providers (ISPs) and blacklist service providers. They are used to identify spammers and block their emails. A spam trap looks like a regular email address, but it doesn't belong to a real person and can't be used for any communication. Sometimes ISPs use expired email addresses of their users as spam traps.
When you send emails to such email addresses, your sender domain will get spam trap hits and your sender domain will incur negative reputation. Due to this, your emails may be quarantined by the receiving server or they may land in the spam folder.