Hi,
Thank you for posting in Microsoft Community.
It seems that you encounter an Client Error Request failed with status code 500 when trying to customize the text in email notification sent by your Admin about reported malware or phishing email. Let's work on this together in finding the best possible solution to this issue.
To better understand the situation, we would like to ask some questions, such as:
- What Microsoft 365 Version & Edition are you using? About Office: What version of Office am I using? - Microsoft Support
We are not 100% sure if these has something to do with product restriction or not, but if you are using a Office 365 Business license then the best option to do is consult your IT Admin and check what causes this issue. We here in Community forum do not have the power to bypass any process connected or managed by an organization.
Sincerely,
Leonardo_Y