We are phasing out Zendesk. Please see Discourse instead: https://community.getmailspring.com/t/move-transfer-emails-between-connected-accounts/286
Unfortunately, Mailspring doesn't allow you to move emails between connected email accounts. You can drag and drop emails into folders / labels in the left sidebar, and use the folder / label pickers to move conversations, but emails cannot be moved to folders / labels in different accounts.
We're hoping to add support for this behavior in the future. Stay tuned!