Email protocol is a set of rules defined to ensure that emails can be exchanged between various servers and email clients in a standard manner. This ensures that the email is universal and works for all users.
A sender using an Apple email client with a Gmail server can send an email to another user using a Zoho mail server on an Outlook email client. This is possible because the servers and the email clients follow the rules and standards defined by the email protocols.
Consider the difference between sending a message via a messaging platform like WhatsApp and sending an email. When you send a message using WhatsApp, the recipient will also use WhatsApp to read the messages. The server which processes the message is also the WhatsApp server. The same platform is used in the server and the client, and hence the entire flow of data is handled by the serving platform in a custom manner.
In the case of email, the sender, recipients, and servers involved can all be different but then they need to receive the data, decipher the content and render it in the same way the sender has sent it. Email protocols define how the email message has to be encoded, how it needs to be sent, received, rendered, and so on, and hence they are essential. While email protocols make the process behind emails a bit complex, the protocols ensure that email is a standard, reliable, and universal mode of communication.
The common protocols for email delivery are Post Office Protocol (POP), Internet Message Access Protocol (IMAP), and Simple Mail Transfer Protocol (SMTP). Each of these protocols has a standard methodology to deal with the emails and also has defined functions.
POP stands for Post Office Protocol. Email clients use the POP protocol support in the server to download the emails. This is primarily a one-way protocol and does not sync back the emails to the server.
IMAP stands for Internet Message Access Protocol. IMAP Protocol is used to sync the emails in the server with the email clients. It allows two-way sync of emails between the server and the email client, while the emails are stored on the server.
SMTP stands for Simple Mail Transfer Protocol. SMTP is the principal email protocol that is responsible for the transfer of emails between email clients and email servers.
Email clients use Mail Access protocols like the POP/ IMAP protocols to retrieve/ sync emails from the server. Basically, mail access protocols are used to download or sync emails from the server.
Email clients use transfer protocol - the SMTP protocol to transfer/ send emails through the server. These protocols are quintessential to ensure that users have the independence to use the email application of their choice, on their own devices. Email clients depend on these protocols to send/ receive emails using a user account that is created in an email server.