Hi @Guavity
What's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook to the latest version.
Considering that different types of accounts in Outlook 2013 and later versions have different attachment size limits. Try compressing the attachment before sending it to check for discrepancies. What is your account type? (Exchange/IMAP/POP)
Also, this can also be a transient backend error. Please contact the administrator in the Microsoft 365 admin center to check if there is a problem with the EX637339. If so, an alternative option available to you is to reply to the message in its Sent Items folder to send the message with the attachment again.
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