Hello,
I don't know if this has been covered or not I'm the new kid on the block
in the forum at least to my knowledge.
So I guess I'll just dive in here and if I do something wrong let me know.
In another area of this forum there was a thread where a person
posted an image capture of the Leopard & Snow Leopard Mail
Account Create window.
What they were showing was the Check Box for the ( SETUP MANUALLY )
feature was gone in the SL version. I was talking to the Mail Technician
at Apple today and he showed me how to achieve this without the check box.
Right at the first screen where you input your email address and password of the account you want to add, do this.
Hold the ( OPTION KEY DOWN ) then Click on Continue button.
Do this on every window that opens until the end where it says CREATE.
Once your at the CREATE window you can just click create without holding the OPTION KEY down.
So don't sweat the check box being gone in 4.1 the OPTION KEY will allow you to manually setup your new account.
Hope this helps.

I don't know if this has been covered or not I'm the new kid on the block
in the forum at least to my knowledge.
So I guess I'll just dive in here and if I do something wrong let me know.
In another area of this forum there was a thread where a person
posted an image capture of the Leopard & Snow Leopard Mail
Account Create window.
What they were showing was the Check Box for the ( SETUP MANUALLY )
feature was gone in the SL version. I was talking to the Mail Technician
at Apple today and he showed me how to achieve this without the check box.
Right at the first screen where you input your email address and password of the account you want to add, do this.
Hold the ( OPTION KEY DOWN ) then Click on Continue button.
Do this on every window that opens until the end where it says CREATE.
Once your at the CREATE window you can just click create without holding the OPTION KEY down.
So don't sweat the check box being gone in 4.1 the OPTION KEY will allow you to manually setup your new account.
Hope this helps.