In PoliteMail version 4.952+, users now have the ability to enable Graceful Fallback. With Graceful Fallback, if one of your expansion modes fail, you can set it to fallback to a different expansion method.
To enable Graceful Fallback, go to PoliteMail > Preferences. You'll notice there are 3 expansion methods - Graph, EWS, and MAPI. If your organization is not utilizing a particular method, it will appeared grayed out and will not be available for selection.
If the mailing lists fails to expand, fallback will rely on the next method on the server side. For example, if your organization uses Graph, and the list fails to expand with Graph, it will fall back to EWS on the server side for expansion. If that fails, fallback will attempt to expand the list on the client side using Graph, then EWS, and then finally MAPI.
Refer to this chart for list type and compatibility with the current preferred method.
There are a number of reasons why your mailing list has failed to expand and your message hasn't been sent. Here are a few examples and their solutions:
Expanding Dynamic Distribution List with Graph and MAPI only enabled.
1. Configure EWS to expand your Dynamic Distribution lists
2. Convert the Dynamic Distribution list to an Office 365 Dynamic Group
Expanding Distribution Group with members marked as hidden on exchange
1. Setup and Enable Graph as an additional expansion mode