Process Inbound Emails Failed: No mail protocol
At Administer > CiviMail > Mail Accounts (civicrm/admin/mailSettings) you want to see two accounts configured, one for Bounce processing and one for Activity processing



It's possible what you're describing is what happens when there is no account configured for activity processing. If so, the error message is a bit misleading, but I suspect that's what you're seeing. If not, continue :)

Both of these mail accounts require working credentials, see "checking credentials" below.

As well as the username / password correctly set, those mail accounts require a protocol - this is probably IMAP, and probably with SSL. (If your mail provider doesn't use SSL, that's bad IMO.)



Checking credentials

Check the mail logins you have work ASAP. You can see the saved password in the DB or by viewing source when editing the account.

I use fetchmail to verify this from the server, so I can check access works from the server in question as well. For a Gmail account bounce@example.org, this looks like:

fetchmail --ssl --check -u bounce@example.org -p imap imap.gmail.com


You can also check by adding a mail account to your desktop mail client, just remove it afterwards.