[2000] Package type is not correct: trying to send internally to an external address

[aisuneko]

Messages could not be sent from Thunderbird through hydroxide. Log shows:

hydroxide[236447]: 2025/03/22 23:27:37 creating draft message
hydroxide[236447]: 2025/03/22 23:27:38 uploading message body
hydroxide[236447]: 2025/03/22 23:27:41 sending message
hydroxide[236447]: 2025/03/22 23:27:42 request failed: POST https://mail.proton.me/api/messages/<redacted_base64>: [2000] Package type is not correct: trying to send internally to an external address

Sending through the ProtonMail web client works fine.