Hi Cheryl,
Are you using an Outlook 365 client? Is there any detailed error message when the issue occurs? if there is, please provide me with it.
Have you tried sending again?
To check whether this is a client issue, please try exiting Outlook desktop client and see if the message can be sent on your web mail.
Besides, based on my reseach, there is a
limit for maximum number of recipients in a message, so it is also suggested to contact your organization admin for some information (To see if a limit has benn set).
You can also try sending this email in multiple batches to less than 50 people at a time.
In addition, to check whether there is a problem with the email itself, please try sending to these 165 clients with other messages and see if there is any difference.
Hope this can be helpful.
Regards,
Aidan Wang
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