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Step 1.
Open Exchange Management Console -> Server Configuration
Right Click on Public Folder Database-> Properties -> Limits

Change Maximum item size (kb) to 76800 for 75MB file size limit from the default 10240 (kb) = 10 MB

Step 2.
Open cmdlet shell in your Exchange Server 2007

Note: 10240 KB = 10 MB

1. Set-TransportConfig -MaxReceiveSize 60MB -MaxSendSize 60MB
Organizational limits: These apply to all Exchange servers in the Organization. You can set these using the Set-TransportConfig command from the Exchange shell:

2. Set-ReceiveConnector “CONNECTOR NAME” -MaxMessageSize 60Mb
Receive Connector limit: Unlike Exchange Server 2003/2007’s SMTP virtual servers, ReceiveConnectors are only used to receive messages. The maximum message size limit can be different on different Receive Connectors on a Hub Transport or Edge Transport server.

3. Set-SendConnector “CONNECTOR NAME” -MaxMessageSize 60Mb
Send Connector limit: Send Connectors are used for sending outbound messages to the internet or particular address spaces (domains). Edge Transport servers also have a Send Connector to send inbound messages to Hub Transport servers in an AD Site.
How to find your Send Send Connectors name
Organization Configuration -> Hub Transport -> Send Connectors -> “Connector Name Here”

4. Set-Mailbox “Joe Adams” -MaxSendSize 60Mb -MaxReceiveSize 60Mb
Mailbox limit: Individual recipients like mailboxes can have their own limits to bypass the Organizational limits. To set these using the Exhange console: Recipients | Mailbox -> select mailbox | properties | Mail Flow Settings tab | Message Size Restrictions.

Other Tips:

Use the Exchange Management Shell to view existing organization message size limits

Run the following command: Get-TransportConfig | fl max*

Record the displayed values of the MaxReceiveSize, MaxSendSize, and MaxRecipientEnvelopeLimit parameters.

Please find the below links for your reference

How to Modify Exchange 2003 Global Message Size Limits in Exchange 2007 RTM
http://technet.microsoft.com/en-us/library/bb310771.aspx

Exchange Server 2007: Setting Message Size Limits
http://exchangepedia.com/blog/2007/09/exchange-server-2007-setting-message_10.html

Managing Message Size Limits
http://technet.microsoft.com/en-us/library/bb124345.aspx

How to Modify the Configuration of a Send Connector
http://technet.microsoft.com/en-us/library/aa998836.aspx

How to Modify the Configuration of a Receive Connector
http://technet.microsoft.com/en-us/library/bb123477.aspx