Block delivery with an alert if a message is too large

If Send Later discovers at delivery time that a message is too large for Thunderbird to deliver, i.e., Thunderbird is going to refuse to deliver the message if Send Later puts it in the Outbox and delivery of other messages will also be blocked, then pop up an alert instead of putting the message into the Outbox.

Unfortunately Thunderbird doesn't do this check when the user schedules the message as it arguably should (see bug 1853253, so this is the best we can do.

Note: Downloading and installing send_later_beta.xpi will subscribe you to future beta releases. We love beta-testers! The advantage is that if there's a bug that affects your workflow you'll help find it and get it fixed quickly. The disadvantage is that bugs are a bit more likely in beta releases. You can unsubscribe from beta releases at any time by downloading and installing send_later.xpi or installing from addons.thunderbird.net.