A couple of our clients have asked how to get Outlook to spell check their signatures when sending emails recently. At first I thought this was entirely unnecessary as you should just check the spelling when you create the signature and it shouldn't change after that. But one of these clients has quite a fancy signature consisting of a sidebar and the email message gets typed along side that - as this then falls within the signature block it does not get spell checked.
We did quite a bit of research on this and everything we found said that it is not possible to enable spell checking in signatures in any version of Outlook.
However, I found a workaround. It's really quite simple and I wouldn't normally post something like this here, but as everyone else is saying it is impossible I thought I really should point out how to do it.
Use stationery instead.
That's it, just move the signature html file from the signature folder under the user profile and place it in the stationery folder. Then disable the signature and go into Stationery and set this as your theme instead.
And there you go, the "signature" is now spell checked correctly.