If User A creates a markup in the ArcGIS Field Maps mobile app and sends it to User B via email, User B can then make edits and additions to the markup. However, when User B sends the markup back to User A via email, ArcGIS Field Maps freezes, and the markup does not open.
This issue is not reproducible in ArcGIS Field Maps. If the issue occurs, contact Esri Support Services, as additional information is required for further investigation.