Which mail providers work with it?
Gmail, Outlook, and any standard IMAP mailbox: Fastmail, GMX, Posteo, mailbox.org, iCloud, your own server. If your mail app can reach it, Pretty Inbox can too.
How do I connect a Gmail account?
Sign in with Google when adding the account; Pretty Inbox requests read and move permissions only. Alternatively, enable IMAP in Gmail's settings and use an app password.
What happens to the mail I clean up?
It moves to your mailbox's Trash folder on the server. Nothing is deleted. Your provider keeps it there (Gmail: 30 days by default) until you empty the Trash yourself, and you can restore any of it in the meantime.
Is it a subscription?
No. Pretty Inbox is a one-time purchase. You buy it once, and it is yours.