Mail Drop Stuck in Outbox

You might try sharing the file with iCloud Drive, DropBox or Google Drive.

How to use iCloud Drive's new sharing features - CNET

The outbox is a queue where large files or multiple files queue up to be processed out. It's like a funnel, only so many bits can be sent up the connection at a time so large files and multiple files queue up in the outbox until the line can process them. So it is normal for large files to clear the outbox slower than small files. You also see files backup or become slow processing through the outbox if a node between you and the email server becomes slow for some reason.

Read this article Add attachments to your emails - Apple Support

Send large attachments using Mail Drop

You can use Mail Drop to send files that exceed the maximum size allowed by your email account provider. Mail Drop uploads large attachments to iCloud, where they’re encrypted and stored for up to 30 days.

• If you have an iCloud account and you’re signed in to iCloud when you click Send, Mail automatically sends the attachments using Mail Drop. Mail Drop attachments don’t count against your iCloud storage.
• If you don’t have an iCloud account, or if you’re not signed in, Mail asks you whether to use Mail Drop (to always use Mail Drop select “Don’t ask again for this account”).

If a recipient uses Mail in OS X v10.10 or later, the attachments are included in your message. For other recipients, your message includes links for downloading the attachments, and their expiration date.

You can turn Mail Drop on or off for an account. Choose Mail > Preferences, click Accounts, select the account, click Advanced, then select or deselect “Send large attachments with Mail Drop.”

It's possible 10.13.1 will fix this problem. Rumors are it should be soon.

Quit Mail.

Go to your User's Library (see below)

Scroll down to Mail folder

In V5, you'll see several alphanumeric folders. Look in each until you see the Outbox.

Drill down in the Outbox.

The Outbox should just contain info.plist. If you see other folders like this...

You can check in these folder but they should all be empty. I'm not sure why the Outbox keeps the folder structure for messages because they are never kept in the Outbox. You can delete everything in Data since it should be empty.

This could improve your problem with the large attachements, but I suspect it's a bug in Mail. If it is a bug, you would be better off using DropBox or iCloud to share the files than trying to use Mail Drop.

You should report to Apple if it's not fixed in 10.13.1

BugReporter
http://bugreporter.apple.com

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Mac OS X Feedback (no account needed)

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Hello !

donkadoo

Perfect, worked well thanks.

Why do you want to sync the outbox? The outbox is just a temporary status indicator, nothing is stored there that needs syncing. If you are offline an email may temporarily reside in the outbox but as soon as you connect again the outbox is processed. If you have the sentbox synced then you know the email was sent and you know it went through the outbox.

You only need to disconnect from the Internet to clear the outbox if an email is hung up in the outbox. Normally email is processed through the outbox immediately when sent but occasionally there is a hiccup and the outbox needs to be cleared manually.

Disconnect from the Internet then delete the email from the outbox. While you are connected to the Internet the email in the outbox will continue trying to send so therefore can not be deleted.