Cannot send large files via Mail Drop on Mail OS X High Sierra

See if any of this applies.

Mail Drop limits - Apple Support

Using iCloud mail the the size of incoming and out coming messages is ( 20 MB up to 5 GB with mail drop turned on ) .

And if you use gmail services see this article Send attachments with your Gmail message - Computer - Gmail Help

Attachment size limit

You can send up to 25 MB in attachments. If you have more than one attachment, they can't add up to more than 25 MB.

If your file is greater than 25 MB, Gmail automatically adds a Google Drive link in the email instead of including it as an attachment. The person you send the file to can download it so they'll have access to it even if you remove it from Google Drive .

Make sure you have checked the box of automatically try sending later if outgoing server is unavailable ( go mail preferences - General you will find this setting )

FWIW, I just sent a 238MB file as a test and it sent fine. I don't get a popup either. You shouldn't if you have Mail > Preferences > Accounts and the email account you are sending from is selected and "Send large attachments with Mail Drop" is enabled. It only asks if it's not enabled.

I'm not sure what the problem might be. I rarely use Mail but it's working for me.

I'm using the identical iMac although I have more RAM.

Try while logged into Safe Mode.

Use safe mode to isolate issues with your Mac - Apple Support

See this article iCloud: Open or download email attachments in iCloud

Large attachments can be delivered by Mail Drop if the sender has turned it on and is using Mail on iCloud.com. You do not need to have Mail Drop turned on to receive these attachments. You can download attachments from Mail Drop for 30 days after they’re sent. - this article for receiver .

Read these articles also iCloud: Add an email attachment in iCloud Mail

Add attachments to emails in Mail on Mac - Apple Support

Make sure you do not use any kind of cleaning apps , anti virus , unknown softwares in Mac , these can corrupt mail or other inbuilt applications and system files .

If you send an email with large attachments , it might be returned by the recipients email service because your email exceeds the size limit set by that email service .

You can try sending the contents of your message in multiple messages or using compression software , such as the compress feature on a Mac , to make your attachments as small as possible or send a message without attachment .The maximum size varies by service provider .

Also , your internet service provider might be blocking the protocol used to send email ( SMTP ) across their network , contact your service provider and ask about SMTP traffic that crosses their network to an external service , such as iCloud , if your service provider blocks SMTP traffic , ask them how you can be exempted from SMTP blocking .