Final answer:
POST3 is the email protocol that removes mail from the server, often causing emails to be missing if accessed from a different device without appropriate settings.
Step-by-step explanation:
The protocol that is known for removing mail from a mail server when emails are accessed is C) POP3 (Post Office Protocol version 3). Unlike IMAP (Internet Message Access Protocol), which keeps the email on the server, POP3 typically deletes the email once it is downloaded to a local device unless configuration settings are adjusted to leave a copy on the server. SMTP (Simple Mail Transfer Protocol) is used for sending emails, not receiving, whereas HTTP (Hyper Text Transfer Protocol) is generally used for accessing information over the internet, not specifically for email retrieval.