First, I'd consider being more explicit than yes/no, try to semantically enhance your buttons and maybe even add an uncertainty element ("don't know" or the likes)
Second, see, how you mentioned "yes", then "no"? Well, IN YOUR CASE this is how it should work, you're using natural language applied to your layout, so people are used to read yes / no rather than no / yes, and therefore doing the latter creates a friction.
Now, there's a very interesting article by Nielsen, where he explains what I said above and mentions its counterpart: placing the affirmative action on the right has a logical flow, as if you're continuing to next step. Quite honestly, I use this approach a lot, but again, IN YOUR CASE you're mentioning this is for an email. In an email, there's no continuation for your action (OK, there might be, but in a browser, so there's an interruption in the flow), so the yes/no is a more logical option.
Also, what are you trying to target? From the above article:
Inconsistency Costs More Time than It Saves
Deviate from the standard, and you'll easily cost users several
minutes — or possibly hours — as they overlook or misuse UI elements.
The time people spend pondering inconsistencies typically sums to much
more than the small savings you'll hypothetically derive from a
Sadly, the Windows Vista User Experience Guidelines differ from the
Apple Human Interface Guidelines when it comes to the sequence of
Windows puts OK first
Apple puts OK last
If you're designing a desktop application for one of these two
personal computer platforms, your choice is easy: Do what the platform
owner tells you to do.
For example, if your target is Windows users, see the recommendations at Windows UX. Now see how Android plays with the continuity concept placing the affirmative button on right , and same goes for iOS
When the most likely button performs a nondestructive action, it should be on the right in a two-button alert. The button that cancels
this action should be on the left.
When the most likely button performs a destructive action, it should be on the left in a two-button alert. The button that cancels
this action should be on the right.
So, we get back to 0.
Now, consider block placement:
Now consider color:
Now, consider choices: is this a neutral choice or do you want to lead the user to some action? If so, color, size, placement and even copy plays a big role!
There's not a set rule for this, so you'll need to consider the pros and cons, natural language, device you want to target and many other aspects. The good news is email tracking is really easy, so maybe you could do a few small batches and then get to a final version that works better for your users