we recently migrated from a shared tenant to ourer own ( Company policy change of ownership)
Prior to this external chat worked flawless - and we have ourer own shared Sfb envorinemtn in ourer datacenter.
After htis migration we encounter only 2 user are able to communicate with external parties (everything and everyone)
Rest just get a message very quickly to state "There was a problem sending this message.
We have a case with microsoft going for around 1 month by now.
We have phased out skype for business pointed all DNS records to 365.
We are really out of context here what the next step is - we are as a business HEAVY