Well it works via Webmail but we couldn’t get it to work via client connections, so we had to pull the trigger regardless and just deal with the migration overhead.
The bigger problem ended up being the migration process itself, it keeps getting snagged on folders and crashing out without detail citing: " user ‘xxxxxx’ has been migrated with errors ", and we can delete those offending files on the source server in some cases and migration continues but sometimes it snags regardless and can’t be mitigated. Migration source is postfix and legacy going back 20 years in some cases so any possible recommendation on tools to solve that would be great 
Thanks guys!
Brad.