Final answer:
Email is predominantly sent using SMTP, with POP or IMAP for retrieval. However, the notion of 'private' email is misleading, as it could be divulged without consent once sent.
Step-by-step explanation:
The most prevalent protocol by which email is sent is the Simple Mail Transfer Protocol (SMTP). This protocol is used for sending emails across the Internet. In conjunction with SMTP, protocols like Post Office Protocol (POP) or Internet Message Access Protocol (IMAP) are often used to retrieve those emails at the recipient's side. SMTP is responsible for the actual sending process, while POP and IMAP deal with the retrieval and storage of emails by the recipient.
When discussing professional vs. private email, it's crucial to understand that in terms of transmission, both use the same protocols. However, the notion of a "private" email can be misleading. Despite potential expectations of privacy, once an email is sent, it could be forwarded, shared, or otherwise exposed beyond the initial recipient. This emphasizes the importance of being cautious with content shared over email, regardless of whether the communication is professional or personal.