I've noticed that Litmus newsletters are not centered in few e-mail clients. It happens in Outlook Web App via Microsoft Exchange or Yahoo.com. I think that adding table-layout:fixed on parent full width tables is not enough to support centering content tables in all clients.

Solution should be to add margin: 0 auto 0 auto; to every content table with set width (e.g. 500px width tables in the last Litmus newsletter).
Also, if you want to center content tables, some clients support attribute align, so what don't use align="center" too.

In my opinion, full code of centered content table should look like this:

<table width="500" border="0" cellpadding="0" cellspacing="0" align="center" style="margin: 0 auto 0 auto; width: 500px;">

Let's discuss it :)