Can't quite answer your question, but I can report that in the last couple of weeks I have found that my posts from gmail subscriptions to a number of mail lists at Google and Yahoo have ceased to be transmitted to the groups concerned, while posts from a Yahoo mail address have continued to work.
Must get round to investigating some time. The point being it may be a GMail issue rather than a Nabble one.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.