This issue is/was being reported (at 30 December 2014) with Apple's iCloud email services, Microsoft's Office 365 email platform amongst others routing these errors back to end users. A thread here explains this;
If you telnet into Apples services you will see the 250-DSN flag being offered;
george:etc dbanes$ telnet mx1.mail.icloud.com 25
Connected to mx1.mail.icloud.com.
Escape character is '^]'.
220 nk11p00mm-smtpin010.mac.com -- Server ESMTP (Oracle Communications Messaging Server 22.214.171.124.0 64bit (built Aug 27 2014))
250 SIZE 28311552
These bounce reports (as link above) indicate that using DSN is causing Apples mail platform to generate this error which results in a bounce.
DSN is explained here;
This is not Proofpoint Essentials (or any other managed email security service) issue, who are just routing the responses between Apple and your mailbox provider (e.g. Microsoft Office 365). Our recommendation is to contact Apple.