In a mixed Microsoft Exchange Server 2010 and Microsoft Exchange Server 2003 environment, an Exchange Server 2010 Client Access server handles the proxy requests for Exchange Server 2003 servers. A user accesses a mailbox which is hosted on an Exchange Server 2003 server by using an IMAP4 client. In this situation, the Exchange Server 2010 Client Access server advertises that the "Literal+" command is supported during the "capabilities" broadcast to the IMAP 4 client. However, the IMAP4 client may encounter certain issues. For example, the IMAP4 client cannot send an email message that has a large attachment.
This problem occurs because Exchange Server 2003 has issues with the "Literal+" support.
To resolve this problem, install the following update rollup:
2407028 Description of Update Rollup 1 for Exchange Server 2010 Service Pack 1
After you apply this update, you can set the DisableLiteralPlus key in the Microsoft.Exchange.Imap4.exe.config file to enable or disable the "Literal+" support feature. To do this, follow these steps:
On the Exchange Server 2010 Client Access server, locate the following folder: