MassMessage requires a preview step before sending.
Preview returns the attached screenshot. with the error message copied here:
Request from 2601:243:402:6830:61b0:b08b:f43d:c32c via cp2019 cp2019, Varnish XID 177303312
Error: 500, Internal Server Error at Thu, 15 Feb 2018 22:20:49 GMT
MassMessage is unable to complete the sending process without completing preview, so this breaks the tool.