Mail Universal Distribution Group rejects external senders

I have a newly created Mail Universal Distribution Group which works fine for internal senders, but which rejects external senders. I know of the authentication checkbox and it is not checked:

[PS] C:\Windows\system32>Get-DistributionGroup -Identity bs2013 | Select-Object RequireSenderAuthenticationEnabled

                                                                                     RequireSenderAuthenticationEnabled
                                                                                     ----------------------------------
                                                                                                                  False

Our Exchange (version 14.02.0342.003) issues the following error message:

550 5.1.1 <bs2013@...>: Recipient address rejected:

undeliverable address: host ...[...]

said: 550 5.1.1 User unknown (in reply to RCPT TO command)

I have also temporarily disabled "Recipient Filtering" in the Anti-spam settings of the organisation's Hub Transport configuration, but this did not have an effect either.

Any ideas what I should look for?

Thanks in advance,
Christoph