How words like "email" have changed pronunciation

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If I understand right, you're asking about the change in pronunciation of words like "illegal" and not of words like "email", right? Also, I'm going to assume North American English, but please specify if this isn't what you had in mind—I'm sure the answer will be different outside of this context.

Assuming I guessed the above correctly, maybe this will help: I heard words like "illegal", "elocution", "equine", and "experiment" pronounced with [i] (for the bolded parts) as early as the mid-'90s (when I started noticing variation of this sort). More to the point, while I don't pronounce any of those words with [i] myself, a lot of these words have known variation—i.e., a dictionary's pronunciation key for most of them will give multiple options, or there's dialectal variation involved. I doubt any of this has anything to do with the "e-" prefix. On the other hand, such changes could happen, and may fall under the category of analogical change.

Are words like "imminent", "illustrate", and "issue" (all with [ɪ] for me) included in the list of words that you think may have changed? If not, why do you think they may be different from "illegal"? (One theory might be that unstressed /ɪ/ before /l/ is [i]-like in some varieties of English, whereas other instances of /ɪ/ are not. Or maybe it's conditioned by the following /i/—which would be a form of vowel harmony?)