I (already!) got feedback from the fastmail support team.
They gave me the following answer:
Quote:
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You can control this behavior using a setting. On your app Tap More->Settings and go-to Preferences. Tap on show advanced preferences and change the "Navigation" settings to Mailbox/Next?previous Conversation.
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While it isn't as flexible as the solution I had in mind, changing the setting from "next conversation" to "previous conversation" at least improves the situation for me.
I do wonder however, who will find this very hidden preference by its own.