Overview + Setup Information
This issue relates to Exchange, SharePoint Online and Office 365.
Here's a quick summary of our setup:
We've integrated our on-prem AD with Azure AD via Azure AD Connect.We have hybrid Exchange set up, with some mailboxes hosted on-prem (Exchange Server 2010 SP3) and others hosted on Exchange Online.Inbound mail flow is directed to Exchange Online so that we can use EOP for anti-spam and anti-malware protection.Issue
Users with on-prem mailboxes are unable to send emails to distribution groups using the 'Send by Email' functionality in SharePoint Online sites.
These users are able to select the distribution group and send the email, however, the message is not received by any of the members of the distribution group.
Solution
Disabling the 'Require that all senders are authenticated' option in EMC ...