Microsoft Exchange 4.x and 5.x allow a remote attacker to relay email (i.e. spam) using encapsulated SMTP addresses, even if the anti-relaying features are enabled.
An encapsulated address looks like this: IMCEASMTP-encapsulatedaddress@yourdomain.com
where encapsulatedaddress is the encoded destination email address. The encoding scheme is as follows: replace "@" with "+40", replace "." with "+2E".
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