Error occurred while trying to send email.

Problem 

When running Outlook on a Terminal Server with Cached mode switched off the following error occurs when initiating an email from LawMaster 

Error occurred while trying to send email. 

The function cannot be performed because the message has been changed. 

Do you want to send the the message anyway? 

Solution 

Set Exchange to use cached mode. 

Some notes below from https://docs.microsoft.com/en-us/exchange/outlook/cached-exchange-mode as a reference:

• Outlook 2016 supports running in Cached Exchange Mode in a Remote Desktop Services (RDS), formerly known as Terminal Services, environment that has multiple users. When you configure a computer running RDS to use Cached Exchange Mode, be sure to consider the additional storage space and disk I/O that are required for multiple client access. New Exchange accounts set up on computers running RDS use Online Mode by default. At setup, the user can decide to enable Cached Exchange Mode. 
• The maximum size for .ost files is configurable. The default is 50 GB of data storage. Make sure that users' .ost files are located in a folder that has sufficient disk space to accommodate users' mailboxes. 
• For more information about how to deploy .ost files to a location other than the default location, see To configure a default .ost location by using Group Policy 
• For more information about how to configure .ost file size, see How to configure the size limit for both (.pst) and (.ost) files in Outlook.