Incoming SPAM Email from Gmail

hemen1645 

Since this is Gmail user, you may file a abuse report directly to Google , so they could investigate the account and they will find with similar behavior, pattern, IP and other methods to block user:

https://support.google.com/mail/contact/abuse 

Meanwhile, ask your users to mark this as Junk, so Anti-Spam algorithm in Microsoft Outlook could identify spam pattern and learn to block it.

You may use admin submission in Microsoft 365 portal and submit some sample of it:

https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/admin-submission?view=o365-worldwide 

You will find some useful resources here:

https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/anti-spam-protection?view=o365-worldwide