This question is similar to \"Standard way to detect mobile browsers in a web application based on the http request\" except for mail clients. For instance, if an email message
You can try to apply @media css queries that target specific browsers like mobile devices. There is a good introduction on the campaignmonitor help website but be aware, it probably is only supported in a hand full of browsers and devices, iOS being on of them luckily :)
Basically you are defining css styles that target specific screen widths so that you can optimize your email for limited screen space.
@media only screen and (max-device-width: 480px) { ... }
When talking really detection and displaying a totally different email, that's really impossible since you are talking about javascript there and that's not done in emails and probably won't even work in 99% of all email clients. But you can go a loooong way with @media queries.