I'd follow the legal document here.
If You Distribute, or Publicly Perform the Work or any Adaptations or Collections, You must, unless a request has been made pursuant to Section 4(a), keep intact all copyright notices for the Work
If the templates do contain a client-rendered copyright notice, then you must keep that in the same form. If they do not, then I see no reason you should have to add them: if the author wanted that information rendered for every client, they could have included said notice.
and provide, reasonable to the medium or means You are utilizing:
What is “reasonable” for an email? I'd say anything which takes up too much space in the rendering may not be reasonable. But I'd also not use comments, since some automatic transformation steps may remove these. Instead I'd go for HTML
<meta> tags, as conveying this kind of information is one of the main purposes of meta tags.
(i) the name of the Original Author […]; (ii) the title of the Work[…]; (iii) […] the URI […]; and (iv), consistent with Section 3(b), in the case of an Adaptation, a credit identifying the use of the Work in the Adaptation (e.g., "French translation of the Work by Original Author," or "Screenplay based on original Work by Original Author").
That lists what you have to include. The URI should go in a
<link> tag, not a
The credit required by this Section 4(c) may be implemented in any reasonable manner; provided, however, that in the case of a Adaptation or Collection, at a minimum such credit will appear, if a credit for all contributing authors of the Adaptation or Collection appears, then as part of these credits and in a manner at least as prominent as the credits for the other contributing authors.
So if you include your own name or the name or the projectas part of the template (as opposed to a signature, which I would consider part of the textual content but not of the template), then you have to name the original creator or project as prominently.
Of course, you can avoid a lot of worries by simply asking the original author which form of attribution they'd like to see, or whether they agree with your preferred form (be it comments or
<meta> tags). Chances are they will be delighted to see someone making use of their liberally licensed content.