The first item to notice is that there are many emails within the email logs that have State as Ready. The second item to notice is that within the Email Diagnostics module the processing time is in red and growing.
Office 365 has limits set in place for how many emails are sent at one time. The error states "Office 365 [Exchange Online] has a 30 message/minute recipient quota, and 10,000 messages /day quota. Exceeding these limits will result in a “550 5.2.2 Submission quota exceeded”
If you believe that your volume may approach those limits your Office365 admin should look into preventing that happening. Use the article provided in Additional Information section which is owned by Microsoft to understand these limits.
1) Check Email accounts to see if you are using Microsoft Office 365 Email servers and not ServiceNow-provided email accounts.
2) Check Email Diagnostics module to see if the SMTP Sender Processing Time is high.
3) Search the Email logs using this filter All > Created on Today > State = Ready > Message ID = NULL
4) Reach out to internal email admin to address the limits in using O365 email server