If you delete a mail route or remove an airship from a mail route, it is impossible to remove or discard any mail still on the ship. The only option you have is to sell the ship to an npc trader and construct a new ship.
Setting up a new trade route didn't work either due to how mail cargo is handled.
@nowayy Hey! Thanks for letting us know about this issue. Would you be able to provide a video showing this so we can take a closer look? We'd also be able to pass the video along to the dev team so they can investigate this further. Once you have the video clip, please upload it to a website like YouTube or Google Drive (Twitter works too!) and share the link with us here.
Thanks in advance!
@nowayy Hi hi, that's totally fine! Get well soon
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