Hi danny satterfield,
I saw your post about Mail Drop, and I'm happy to help out.
Testing using both the Mail app and webmail helps to narrow things down. This article has a few points on Mail Drop limits that may apply to your situation:
"Attachments might not be sent using Mail Drop if they exceed these limits:
The message, including its attachments, is larger than 5 GB. Try sending the contents of your message in multiple email messages or compressing your attachments to make them as small as possible.
An uncompressed folder is attached to the message. Use these steps to compress your attachments.
Too many messages have been sent or the maximum number of recipients has been exceeded. Learn about mailbox size and message sending limits.
You reached the 1 TB Mail Drop storage limit. Each attachment expires after 30 days, so you can send new attachments after earlier attachments expire and more storage becomes available."
Do any of these apply?
If not, are you able to send a Mail Drop attachment using a different platform, like an iOS device?
If you're able to send from an iOS device, this link would be a great next step to test further on the Mac end: How to test an issue in another user account on your Mac. This will tell you if the issue is user account-specific or system-wide, and the link has additional steps you can try depending on the results.
I hope this helps get you started.
Cheers.