We have a web based on-send Outlook add-in (not Smart Alerts) that works well.
Our client uses Proofpoint, an Outlook COM add-in that appends "[secure]" into the subject of the outgoing email under certain circumstances (like if there's an attachment).
The two add-ins work fine separately, but when used together, the Proofpoint COM add-in fails to insert the "[secure]" keyword into the subject of the outgoing email. Our web based on-send add-in is called normally.
How can we diagnose what is going wrong? Is there a defined order for which add-ins run when (for example, do all COM Add-Ins always run first)? Can multiple add-ins both make modifications to the outgoing email?
You can't be responsible for other software vendors, only Proofpoint developers can troubleshoot issues like that. There is no explicit reason why a particular add-in fails to do its job in Outlook.
And yes, all add-ins are run one after another (sequentially), so each add-in in the chain can make modifications to the item. In your case I'd suggest contacting Proofpoint developers for troubleshooting the issue with their software. Nobody else can help.
You may try to create a sample COM add-in where in the ItemSend event handler you can modify the Subject line and see whether it works with your web add-in. If that works correctly - the issue is on the Proofpoint side.