Unless your email marketing campaign is specifically engineered for the less-than-25 age group, you should take pay particular attention to the needs of seniors.
Representing the fastest growing purchasing power, seniors account for millions in spending every year, potentially on some of your products or services. Remember that our youth-enamored culture pushes older folks to act "young": some products targeted at the less-than-40 crowd attracts a large portion of seniors.
Especially with a rapidly aging population, special needs of seniors must be taken into account. Primary factors to keep in mind are vision and motor skills. If you are truly committed to your email marketing efforts, wake yourself up after an hour or two asleep and read your newsletter as it will appear to prospects and clients. The grumpier you are, the better: this is when the most irritating of all situations jumps at you the clearest.
Regardless of their current "tech-savviness", keep in mind that seniors have had to adapt to a brand new tool and methods of communicating: always assume that they do not enjoy particularly staying in front of a computer: keep your newsletters short and to the point, and include a link to your website for every topic that warrants more explanations: interested readers will click on the link to learn more, and others will just skip to the next paragraph in their email message!
For anyone younger than forty, with a good vision, font sizes of 10 or 11 pixels are not a problem. For an older audience, however, deciphering a newsletter written at 10 pixels font size can be painstakingly wasteful. If you have a mixed age-group to which you send your marketing emails, better opt on the safe side and choose a large(r) font size, like 13 pixels. For email marketing designed for seniors only, you may as well use 15 pixel high letters: this size might look strangely "loud" to a young audience, so reserve it for seniors-only newsletters.
Another aspect of email marketing particular to seniors (which applies to web design as well) are the color contrasts used in your newsletter. The older we get, the harder it becomes to distinguish lettering over a similar toned background. You need not stick to primary colors, but avoid too subtle contrasts: many seniors would not even realize that this beige block on the left contains darker text.
Arthritis affects most seniors, and can make the use of a mouse or pointing device quite challenging. The same website accessibility guidelines also have their raison d'etre in senior-targeted email marketing.
Make buttons, links and other controls large enough to be easily seen, but also large enough to be easily clicked; simply by following our advice on font sizes, your inline links will already be a lot easier to spot and click.
Image links and banners should follow the same guidelines. Clickable banners using type on photographic backgrounds should use a high enough contrast to be legible: if your branding doesn't accommodate well high contrasts (if it uses muted tones, for example), resort to dark outlines or shadows to create the visual contrast.