Outgoing mails with attached photos are only sent when phone is connected to power supply (charger)

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App

Thunderbird for Android

App version

11.1

Where did you get the app from?

Google Play

Android version

15

Device model

Samsung Galaxy A35 5G

Steps to reproduce

1. Click "Compose"/"New message" button (pencil icon)
2. Enter recipient (e.g. myself)
3. Enter any subject (e.g. "Test")
4. Click attachment button (paperclip)
5. Select a photo from gallery => it's shown as an attachment in the mail currently composed - this one is 3.2 MB big
6. Click "Send" button (right arrow)
7. Wait => even after enough time, the mail to myself does not arrive in my inbox
8. Connect phone to charger.
9. Go to outbox.
10. Press "Send messages" => sometimes the messages are sent, sometimes they aren't.
11. Force-quit Thunderbird app, go to outbox again, press "Send messages" => sometimes the messages are sent, sometimes they aren't.
12. Compose new message from message in the outbox and send it => now it's sent reliably.

Expected behavior

• Messages should be sent reliably even when not connected to a power supply, at least with decent battery levels (always > 60% when I tried it out)
• If messages are not sent immediately, there should be some sort of indication of that, with the reason
• And if everything else fails, one tap on "Send messages" in the outbox should be enough to reliably send the messages

Actual behavior

Messages with attachments of a few MB get stuck in the outbox, and it sometimes takes quite a bit of creativity to actually get them sent ...

Logs

No response

Start by reproducing the issue on Thunderbird for Android 11.1 with Android 15 and a Samsung Galaxy A35 5G, sending a several-megabyte photo attachment while on battery. Inspect the outbox and send flow, comparing behavior before and after connecting power. Done means attachments send reliably without charging, or the outbox clearly reports why sending is delayed.