we can retrieve contributor handle by email?

Fetching contributors like that will only work if your user ids are actually emails. If you have other user ids but have access to their emails, you have to use a different approach.

This is sort-of pseudo code, but mostly translates to working code. Also, this assumes you're using the server sdk. I've split the functions up to make their intent clear. The first function gets all contributors and iterates over them to find the one matching the email in question. The second function uses framework capabilities to try and retrieve all available contributors. The empty parameters filter nothing, which should return everyone.

public IContributor getByEmail(String email) {
    IContributor[] contributors = getContributors();

    for (IContributor contributor : contributors) {
        if (contributor.getEmailAddress().equalsIgnoreCase(email)) {
            return contributor;
        }
    }
    return null;
}

private IContributor[] getContributors() {
    IContributorRestService.ParmsGetContributors parameter = 
        new IContributorRestService.ParmsGetContributors();

    try {
        return contributorRestService.getContributors(parameter);
    } catch (TeamRepositoryException e) {
        return new IContributor[0];
    }
}

 You can use Query models as explained here to find items by other values than the key.