When you use Administrative Tools / Internet Services Manager to expand the <WebServerName>, right-click Default SMTP Virtual Server, press Properties, select the Messages tab, and clear the Limit message size to (KB) box, the default message size should be 2048 KB. If you do this, when the storage space on the Drop folder exceeds 20 KB, all RCPT TO commands generate 452 4.2.2 Mailbox full?
This problem is caused by an incorrect default value built into the program.
To workaround this problem, check the Limit message size to (KB) box and set the limit manually, by typing 4096, or some other limit. Press Apply, OK, and exit the snap-in.