I have - though only in recent years. When emails began to be widely used, in the mid-1990s, the netiquette manuals stressed the new and informal character of the medium. Formality was to be avoided, so that even a 'Dear' opening was not recommended. However, these manuals were written in the early days of e-communication, and usually by younger and geekier people. As the age demographic of internet users changed, with older (and more conservative) people coming online, so the stylistic range of emails altered, as did the range of contexts in which it was felt emails were (or weren't) appropriate. For some time, for example, it was considered inappropriate to send condolences for a death by email, but this happens now. Similarly, firing people by email was widely criticised a decade ago. Not so much now.
Today we see the whole range of formality in email exchanges - from those that replicate letters in every formal detail to those that avoid all traditional letter-writing conventions. I have had emails beginning 'Dear Professor' and ending 'Yours faithfully', or the like. And mixed styles are encountered too, such as beginning with 'hi' and ending with something more formal. Computer-generated emails often mix things up: I got an email once which began 'Dear Professor Wales'.
It's difficult to work out what is going on because there is so much anonymity 'out there'. Sociolinguists rely on context for their observations - age, gender, language background, and so forth - and this is usually missing or unclear in internet exchanges, especially in forums and social media. So it's often impossible to interpret the factors that have led writers to make their formality decisions. And even if one knew, it would be too soon to generalize, with a medium that is still (for most users) less than twenty years old.