No Mail Drop Option, But It's Selected For Large Attachments

When I attempt to send an email with numerous photos (Large Selected), and the entire message is oversize, there is no Mail Drop option to select.
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Below is what I see.
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“Cannot send message using the server iCloud
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Your message has size 29.3 MB, which exceeds 28.3 MB, the maximum allowed by the server.
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Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be sent.”
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This is how my Mac Mail is set up.
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Accounts
iCloud IMAP
Account Information
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‘Enable this account’, is checked.
Status: Online
Download Attachments: All
‘Send large attachments with Mail Drop’, is checked.
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iMac (27-inch, Late 2014)
10.13 (17A405)
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How might I get the opportunity to use Mail Drop?
Is it only for one oversize attachment, and not oversize messages with too many attachments?

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