The correct answer is SMTP. Show Key Points
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SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail. However, since it is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, POP3 (Post Office Protocol) or IMAP (Internet Message Access Protocol), that let the user save messages in a server mailbox and download them periodically from the server. In other words, users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving e-mail. How SMTP worksSMTP works as a three-step process, using a client/server model. First, an e-mail server uses SMTP to send a message from an e-mail client, such as Outlook or Gmail, to an e-mail server. Second, the e-mail server uses SMTP as a relay service to send the e-mail to the receiving e-mail server. Third, the receiving server uses an e-mail client to download incoming mail via IMAP and place it in the inbox of the recipient. SMTP vs. IMAPHosted on an SMTP server, SMTP is used to send, relay or forward messages from a mail client, but cannot receive messages. On the other hand, IMAP is an e-mail protocol that deals with managing and retrieving e-mail messages. IMAP keeps an email on a server, and then synchronizes it across several devices. IMAP is used in receiving emails, not sending them. When used together, SMTP and IMAP transmit email messages. SMTP vs. HTTP APIsSMTP has been used since 1982 and remains the most common e-mail protocol to send an e-mail message nearly four decades later. However, there has been a trend toward using cloud-based HTTP APIs to send and receive e-mail. HTTP APIs offer two advantages: The communication between e-mail client and server (for example, when using mobile apps) is faster than SMTP because HTTP APIs require fewer back-and-forth commands to authenticate the sender and recipient. Additionally, APIs offer functionality that is not available using SMTP. SMTP and Unix-based systemsOn Unix-based systems, Sendmail remains the most widely used SMTP mail server for e-mail. Sendmail provides the-behind-the-scenes e-mail transport and is typically used with a separate user-friendly interface. SMTP usually is implemented to operate over Internet port 25. An alternative to SMTP that is widely used in Europe is X.400/X.500. When it was created, X.400 was intended to serve as the primary international protocol. Among most users, X.400/X.500 has been eclipsed by SMTP, but it remains popular in the defense and telecommunications industries.
When we speak about email accounts, there are multiple protocols that are involved. This can be extremely confusing if you are not aware of what protocols exist for emails. Also, each of these protocols performs slightly different functionality. The commonly used protocols are- IMAP, POP3, SMTP, and Exchange. These are few protocol types one would come across while accessing an email client. The protocol details can be accessed via the server settings based on the email client being used. POP3POP3 stands for Post Office 3 protocol, and it is used for retrieving emails. POP simply reaches out to the mail server and brings back the mail contents. This is a simple yet standardized way that allows users to access mailboxes and quickly download messages to their devices. With POP3, users can configure the server settings. This can be used to allow mail copies to be left on the server or move all emails without leaving any copy on the server. This is usually configurable in most cases. The biggest advantage of POP3 is the low dependency on the Internet. Users can download all emails and read them at leisure even if they are accessing this offline. The way these emails are stored locally depends on the email client. For instance, Outlook utilizes .pst, while Thunderbird uses .mbox. This is a good option in case you choose to read emails offline. Apart from this, this helps you reduce the server space by storing messages locally. The default ports for POP3 are:
IMAPThis stands for Internet Message Access Protocol, and it is also used to retrieve emails. This again is a standard protocol for accessing emails and is a client/server protocol. Here the emails are received and held by the Internet server. Unlike POP, this does not move the emails. The biggest difference between POP3 and IMAP is the mail sync up. POP3 assumes that a user will be connected to a single device. However, IMAP is suitable for different devices simultaneously. IMAP requires users to be constantly connected to the Internet. When a user accesses the mailbox, the user is actually connected to an external server. This is more beneficial when there are multiple users. IMAP can work over a relatively low internet connection since it only downloads email messages from the server when a user has requested to read a specific email. The default ports for IMAP are:
SMTPThis stands for Simple Mail Transfer Protocol. This is a standard protocol for sending emails over the Internet. This is a protocol that is used by a Mail Transfer Agent to deliver emails to a recipient’s email server. This is a protocol that defines mail sending and cannot be used for mail receiving. SMTP is the most commonly used protocol for mail transfer between two servers. This requires no authentication to function, unlike POP3 and IMAP. Certain Internet Service Providers block the default port 25 of SMTP. In such cases, the mail server also provides an alternate secondary port. The default port for SMTP are:
HTTPThis is a commonly known protocol and stands for HyperText Transfer Protocol. This is not an email-specific protocol. However, HTTP is used for email access using web-based emails (i.e. webmail). Hotmail and Gmail are examples of using HTTP as an email client. This is used to compose and retrieve emails from a web-based account. The default port for HTTP are:
Exchange Account (EAS – Exchange ActiveSync)This is used by Exchange servers like Microsoft Exchange. This not only syncs mail but also syncs contacts, calendars, notes, and everything in the outlook. The advantage is that users can have a synced copy of the calendar, contacts over multiple devices.
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