I'm trying to figure out why. Maybe a look at its properties can help...

This email is a personal newsletter, and as such
- has 3 attachments totaling about 4.5 MB in size (1 PDF, 2 images), but is sent via SMTP using Gmail, which should accept 10MB or more.
- has more than 100 recipients, and has spent some time as a draft, being edited and resaved a few times. Most are BCC.
- contains no HTML except for a hyperlink in the signature.

Other emails from the same address send just fine, and it's the first time in years that I've had issues with email sending (thankfully!).

I would just copy everything into Gmails webmail interface, but copying well over 100 recipients by hand would be a chore. Besides, it would be nice to find out why.

I have used akonadiconsole to restart the Akonadi server, to toggle off- and online the mail dispatcher agent, to restart it, to manually synchronize it, no success. (In fact, it was turned online when I first checked.) The mail dispatcher agent doesn't seem be aware of the mail in the outbox.

My software is from the up-to-date stable repo in openSUSE.

I appreciate any help!

What kind of error do you get when you try to send this email?

Absolutely nothing. It just sits there.

Akonadiconsole did report that mysql (the database used) was not found, but the database was running properly nonetheless, and I didn't have this problem with any other email.

Anyway, I ended up finding a convenient way to copy and paste all the recipients to Gmail in bulk from the reading GUI (e.g. when you select the email in your outbox), so I ended up doing that.

Was the number of recipients larger than the one configured in the composer? (Settings > Composer > General)

Good idea, but it does not seem to be the case. I estimate that there cannot be much more than 150 recipients, while the limit is set to 200.

You could restart the PIM system from a terminal ("akonadictl restart" and try to send such a mail, to see if errors appear on the terminal).