Yikes, you would have to find one acceptable with LP otherwise they'll disable it. And, I'm sorry but if you try to install TFMail, I warn you that will be an even bigger headache.
Let's try something ...
Do you have another email account like yahoo.com or something? Cuz, someone had a similar problem with not being able to get the return email with their LP email addy, but was able to get it with their yahoo.com addy.
So, if you do have another email addy we can temporarily substitute it to check out if this is the problem for you, too.