To enable sending as a distribution group in Exchange 2007 / Exchange 2010

EMS:
Add-ADPermission -Identity "group name" -User "user alias" -AccessRights extendedright -ExtendedRights Send-As

Outlook:
To test straight away, Download the offline address book (click Send/Receive tab, click Send/Receive groups and select Download Address Book).
Send test email from distribution list.

Increase MaxHeaderSize on Exchange 2007 and Exchange 2010

Emails with large headers/many recipients bounce to external senders with 4.4.7 Could not deliver the message in the time limit specified.

To check current configuration:
Get-ReceiveConnector | ft name, MaxHeaderSize

To set a new limit:
Set-ReceiveConnector “Windows SBS Internet Receive Servername” –MaxHeaderSize 128KB

Source – Exchange Inbox
Source – Technet

To increase the send and receive limit for all users on Exchange there are 3 settings that need adjusting. Global Transport, Receive Connector, Send Connector.

Global Transport Settings

To check current configuration:
Get-TransportConfig | ft MaxSendSize, MaxReceiveSize

To set a new limit:
Set-TransportConfig –MaxSendSize 30MB –MaxReceiveSize 30MB

Receive Connector Settings

To check current configuration:
Get-ReceiveConnector | ft name, MaxMessageSize

To set a new limit:
Set-ReceiveConnector “Windows SBS Internet Receive Servername” –MaxMessageSize 30MB

Send Connector Settings

To check current configuration:
Get-SendConnector | ft name, MaxMessageSize

To set a new limit:
Set-SendConnector “Windows SBS Internet Send Servername” –MaxMessageSize 30MB

Source: Official SBS Blog