At Exclaimer, we see a lot of email signatures. We see what a lot of companies get right and what they get wrong.
DO write full-length HTML
As with the reasons for using Inline CSS, write out the HTML longhand. A lot of email platforms won’t recognize the shortcuts you’re used to, which will cause image and alignment issues.
For the non-developers among us, that means you shouldn’t try to use a direct marketing email CMS or Word to build your template – use a tool that was made just for email signatures.