For emailing there are two categories:
-Email client
-Pop3 (Post office protocol)
-SMTP (Simple Mail Transfer Protocol)
Advantages:
- Mails are downloaded on PC
- New mails can be created in offline mode
- Saves time and money.

-Web mail service
-IMAP (Internet Mail Access Protocol)
Advantages:
- Global Access.
- The mail server can be directly accessed.

Protocol is a predefined standard set of rules to establish communication between multiple computer or devices.

Advantages of email service:
- The main advantage is its simplicity.
- It is easy to use
- It is fast
- It provides a permanent record of communication.
- It is an informal medium but capable of formality with attachments
- It is capable of delivering almost any message and files type to almost any recipient or group of recipient, provided they are connected to the internet.

Disadvantages of email service:
As usual though this advantages also point to some of the disadvantages.
- It is so easy to use it is often misused.
- At a simple level people sent inappropriate messages.
- People spoof other people’s addresses
o It is easy to write a program that will give a fake address in the “form” field of an email, thus leading the receiver to believe the email is from a trusted source. Email viruses do this routinely.
- Viruses, Worms and Trojans can be attached very easily
- Spam mail can block up mail boxes and whole area of the internet itself.

Reference website:
- Email and how it works