The Recipient’s mailbox cannot receive messages this big in Outlook

SMTP Status 523 : This error will be received when the total size of the message you have sent (ie: message + all of its attachments) exceeds the size limits on the
Recipient’s server. Many companies implement the good practice of configuring their servers with limits on the size of emails they can receive to prevent their systems running out of space as a result of a spam attack where the spam emails contain large attachments, or as a result of valid but not very technically savvy senders sending enormous scans (through not knowing that scanning at 1200dpi rather than the usually perfectly usable and acceptable 300dpi, will create humongous attachments).
Check the size of the email you sent, and, specifically, the size of the attachments you included, and consider splitting your email into smaller emails. If that does not work, check with the Recipient the maximum size of email they can receive, and if that is still prohibitive then consider FTP arrangements between you and the recipient.
SMTP Error 523 is often a secondary SMTP error code rather than a primary error code, as in the following
examples : “SMTP Error 450 5.2.3 Message Size greater than allowed by Remote Host” or “SMTP Error 552 5.2.3
Data size exceeds maximum permitted” or “SMTP Error 552 5.2.3 Message exceeds maximum fixed size”, and soon ....
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