Any potential mechanics lien claimant is entitled to send a Colorado Notice of Intent to Lien if they've gone unpaid for their work. And, to be sure, it should be sent at least 10 days before a mechanics lien would be filed - so it should be sent well before the lien deadline to provide a chance to get paid without having to file a lien. The document shouldn't be sent by email, though - the Notice of Intent must be personally delivered or sent by registered or certified mail with return receipt requested.
If you've got other questions on Colorado's Notice of Intent to Lien requirements, here are two great resources that should answer just about any question you might have on the subject: (1) Colorado Notice of Intent FAQs & Guide; and (2) Colorado Notice of Intent to Lien Legal Questions.