I was tinkering the other day and found this feature when emailing a large iPad screen recording to my Windows PC. I recorded my creation of a Shortcut to find the halfway meeting point between two locations. I then selected the 100 MB video in Photos on my iPad, and tried to mail it to my PC so I could edit it for publishing on my blog.
I knew the file was too large for email, and so did Apple. When I tried to send, a window opened, indicated the message was probably too large for email, and asked if I wanted to use Mail Drop. Before choosing Mail Drop, I canceled sending and decided to add a second video to the email, just to see what would happen. I opted for Mail Drop when i was ready to send, and this is when things got interesting.
I use Outlook for email on my computer. When I received the message in Outlook, I saw five HTML (that's website format) documents attached to the message. There was also a download link for only the first video attachment in the body of the message, with a note the attachment would be available for a date 30 days into the future. This link did open a download web page, but there was no link for the second video in the email body.
I next opened the five HTML docs and found three of them were empty and two actually opened download links at my iCloud account. However, the links showed zero bytes available. I clicked them anyway, and was able to download both videos. I next checked the same email message on my iPad, iPhone, and in my email account online. Both messages had two download links within the email body. I cannot explain why Outlook on my computer behaved so differently than the same message behaved on my iPad, iPhone, and in the online version of the message.
If the Mail Drop option doesn't automatically appear for you, then try these steps:
Log into your iCloud account online at www.icloud.com.
Click the Mail icon.
Click the gear icon in the bottom left corner of the left hand pane and click Preferences.
Click the Composing tab and then check the "Use Mail Drop when sending large attachments" box.
Click Done in the lower right corner and sign out of the account.
I do not have an Apple MAC computer, but this option probably works on it, as well as on the iPad and iPhone.