In Microsoft Exchange 2007 you have 10Mb limit for messages size. To increase this limit, you have to do several configurations.
After setting the single user mailbox limit from the GUI (Exchange Management Console), go to the shell (Exchange Management Shell) and type the commands below. Uppercase commands print the configuration parameters, "set" commands modify the values.
set-transportconfig -MaxReceiveSize VALUE
set-transportconfig -MaxRecipientEnvelopeLimit VALUE
set-transportconfig -MaxSendSize VALUE
set-receiveconnector -MaxMessageSize VALUE (if requested, the "identity" are listed by the upper command)
set-sendconnector -MaxMessageSize VALUE (if requested, the "identity" are listed by the upper command)
VALUE can be expressed, for example, like 10000 or 10MB