We have been receiving calls on emails that are sent from an O365 account (this will affect emails from other email servers as well) to a group of recipients that has more than 1 gmail email address in it are getting bounce back messages from gmail.  But the callers are reporting that they can send to 1 gmail address at a time.

We checked and there are multiple reports on this issue. Google announced it will be bolstering phishing and malware delivery defense in 2024. https://www.bleepingcomputer.com/news/security/google-to-bolster-phishing-and-malware-delivery-defenses-in-2024/#google_vignette After some research it looks like this is already happening, 

The fix is to setup dmarc / dkim settings on your account.  If you need help with this please let us know.  Here is an article on this if you would like to read about it https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/email-authentication-dmarc-configure?view=o365-worldwide