"Message not delivered because of current system limit" error message

If you try to send an e-mail message that has an attachment to an external mail server, the message may be returned and contain a non-delivery report (NDR) that contains the following error message: If you verify that the size of the e-mail message is less than the current system limit, and then you try to send the message and attachment again, you again receive an NDR that states that the message is over the limit.

If you troubleshoot this behavior by using the Microsoft Exchange 2000 Archive Sink utility, you may notice that the message that Internet Mail Service is sending is larger than the message size that is displayed in Microsoft Outlook.

If Internet Mail Service sends e-mail messages to an external server, the messages are encoded in Base64 or Quoted-Printable formats. If messages are encoded by using the Base64 format, the messages can become 133% of their original size. If messages are encoded by using the Quoted-Printable format, the messages can become up to 200% of their original size but typically become 120% of their original size.

The Base64 and Quoted-Printable formats are used to convert 8-bit mail into a 7-bit mail format to support external servers that may only handle ASCII (7-bit) characters. When messages are received, they are decoded by using the inverse process to convert 7-bit messages into 8-bit messages.

NOTE: Messages that are composed by using 7-bit ASCII characters are not modified. 7-bit ASCII characters can be read by all external servers.

This behavior is by design.

About the Base64 format

If messages are encoded in the Base64 format, the size of the message expands proportionally. Three 8-bit characters (24 bits) are mapped to the Base64 character map table, which contains only 64 characters. The 24 bits are separated into four 6-bit chunks. In this equation, 2 raised to the 6th power is 64, which is equal to the number of characters in the Base64 character map table.

The binary value of each of these 6-bit values has a corresponding ASCII character. The ASCII output value is sent as the body of the message. Specifically, 24 bits expand to 32 bits (32/24=1.33). Therefore, a Base64 encoded message is exactly 33% larger than the original message. Routing information is also added to the message header, but the size of this information is negligible.