In Exchange 2007 you can send a message as long as it does not exceed the message size limits configured for you. It does depend on the message size limits set for the intended recipient(s) whether or not the mail will be delivered to its destination.
You can define message size limits in different places: organization wide, connector wide, and mailbox specific. And when you send a mail it's the most restrictive limitation that is considered.
The default organization wide limit is set to 10Mb, both as sending and receiving maximum message size. This can be changed by using the EMS cmdlet Set-TransportConfig and the parameter MaxSendSize and/or MaxReceiveSize, or by using th EMC, and changing the settings you can find via Organization Configuration - Hub Transport - Global Settings - Transport Settings - General.
Another default limit of 10Mb is set on both Receive Connectors created during the installation of the Hub transport server role. This limit can be changed as well using the EMC or using the EMS cmdlet Set-ReceiveConnector and the parameter MaxMessageSize. When creating a send connector, or afterwards, you can define the maximum send size for that send connector, by using the EMC or using the EMS cmdlet Set-SendConnector and the parameter MaxSendSize.
The last place to configure message size limits, is on the mailbox itself. By default there are no message size limits configured for a mailbox-enabled user. Both the EMC and the EMS can be used to define these limits. Using the shell it's the cmdlet Set-Mailbox and the parameters MaxReceiveSize and MaxSendSize, using the console, you can go to the properties of your recipient, select the property page Mail Flow Settings, and select the Properties of Message Size Restrictions.