TE wrote:

Every message I either forward or reply to comes back with the
message:

554 Authentication Failed, must login.

I am not sure what this message is telling me. I am logged in, so I
do not know what it wants me to log in to.

It means that you have not properly configured your Outgoing Server values
properly. You have not checked the box that indicates Outlook should log
into the outgoing server. Since you didn't state your Outlook version,
there's no good way to tell you exactly how to rectify the situation.
--
Brian Tillman [MVP-Outlook]