wentieth century) and it is the basis of personal and business communication (with electronic 6.4 QoS for Popular Internet Services means) in the twenty-first century. As the name (electronic mail) says, it is delivered in the same manner as regular mail, SO the sender creates the email and sends to a recipient, the sender's email server communicates with the recipient's email server, and finally the recipient chooses when to check the mailbox for new emails (with email use on mobile devices, it has become popular to use the push email functionality, which notifies the recipient about new email messages). Considering this, email is not a real-time service. Email protocols use TCP/IP, SO the messages are delivered to the destination email server in a reliable manner, which is based on the TCP protocol reliability. Also, email as a non-real-time service typically is classified with lower priority than real-time services (from the traffic management point of view), and it is served via Internet access service based on the network neutrality principle. However, email functionality on both sides is important. For example, if the email server is down on the recipient's side then the email cannot be delivered (typically the sending email server will periodically try to deliver a given email message until it is several days old). Also, many email servers have a limit on the total size of an email message (that size, measured in bytes, is typically bigger than the size of the email message text and all attachments due to their encoding in the cre- ation of the email message at the sender's side), SO some messages may not be delivered due to their size. Certain email servers may become less responsive due to overload, either with regular messages or due to denial of service (DoS) attacks. Overall, there are a number of standardized QoS parameters for email services [17], which are given in Table 6.4. One should note that such QoS parameters refer to all email protocols in use, including SMTP (Simple Mail Transfer Protocol)