Can I move or transfer emails between connected accounts in Mailspring? Thunderbird let me, so surely Mailspring should too.
No—unfortunately Mailspring does not allow you to move messages / conversations in one email account to another connected account. While this is technically possible and supported by other clients like Thunderbird, Mailspring only allows you to move messages to folders / labels within the same account.
We may add this functionality in the future if it’s a popular request. If this is something you want to see, vote for this feature here: Move emails between accounts