Archived emails can always be retrieved. There are two ways:
1. If you realise just after you archive it, swipe back, tap again and you will see the "archive" button highlighted. Just tap it again to undo.
2. If it's a bit later, go into your regular mail program, for example Gmail, Outlook, or your webmail. The location of your archived mail is different for gmail that other mail servers.
Go to Gmail in your browser and go into the folder called "all mail" and you will see everything including archived emails. Or you can search in gmail.
Archived email will be moved to a folder called "archive". This will either be a built in folder that already exists or Speaking Email will create it as a personal folder.