Using classic Outlook for Windows in business environments
Hi @James, Randy
Thank you for reaching out to our Q&A forum.
May I know which method do you use to connect this account? (Exchange or IMAP/POP)
Can you please confirm you are seeing this error message?
If yes, normally Outlook will store the email message in Outbox. Please have a check there. You can see the size limit is 151MB. It exceeded the limit which is 150MB (this applies for Exchange Online; the limit might be different if you have another provider)
You can read here for more details Exchange Online limits - Service Descriptions | Microsoft Learn
You can consider reduce the size of the attachment to prevent this issue.
Reduce attachment size to send large files with Outlook - Microsoft Support
I hope this information helps.
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