I have seen some of the websites who are using
nofollow at mail address.
I want to know about it that if I use it so will I get any benefit from SEO purpose.
Checking the official doc:
"Nofollow" provides a way for webmasters to tell search engines "Don't follow links on this page" or "Don't follow this specific link." [...]. It is used for untrusted content, paid links or crawl prioritization.
Having in mind crawlers don't follow
mailto links, I guess the main point for using that is to prioritize the important links and avoid a waste of time.
Google probably doesn't crawl mailto: links as it knows there is nothing to crawl. And mailto: links probably don't pass any link juice either. But because there is a possibility that Google might pass the email addresses your link juice, it is worth adding the nofollow tags to these links unless you are 100% sure otherwise.
There isn't any risk of adding nofollow to links that you don't want Google crawling or passing link juice to. And in this case, you wouldn't want Googlebot to crawl the email addresses or pass them link juice. So if there is any flaw in the Googlebot algorithm you're helping it to understand and interpret the link by adding nofollow to it.
Googlebot wouldn't want to crawl the email addresses as it is a waste of crawl bandwidth, but if for some reason its code causes it to crawl it then you're helping it by telling it not to.
But most importantly you're ensuring that you're not passing your link juice to these links, which is worth the few extra bytes of bandwidth that the html code adds.