Recipient SendAs:You can't use the domain because it's not an accepted domain for your organization

Please refer the below article

https://technet.microsoft.com/en-in/library/dn249373(v=exchg.150).aspx

Using the above article we have already implemented and also we are syncing the mail enabled public folder to exchange online using the below link

https://blogs.technet.microsoft.com/undocumentedfeatures/2017/04/08/migrating-hybrid-public-folders-to-office-365/

After the above setup, we are able to give send as permission on mail public folders to the users, but now we are facing issue only on a particular mail enabled public folder and the error is quite irrelevant