When you understand that your sign-off is simply wrapping up the message, it's a little easier to choose which one is appropriate. For example, a sales pitch and an email about an interview both demand a reply. But, you want to be friendly and professional. A sign-off like TTYL just won't do because it's unprofessional, while 'Always yours' can be creepy and overly friendly. This type of communication is where expressing gratitude for the recipient's time and letting them know you look forward to hearing from them comes in handy. If you are sending an email to someone of high authority (with whom you would not have water cooler conversations), professionalism is key. 'Respectfully yours' is appropriate here, as is 'Warm Regards,' or 'Sincerely.' Emails to your bosses and co-workers can be a tad tricky. First, you don't want to send any written communications that can lead to a call to HR. Second, you don't want to be overly formal or too relaxed. The safest option for these relationships is "thank you for your time" or "please reach out to me for further questions."