A mail user agent is also known as an email client; it is a computer program that allows you to send and retrieve emails by interacting with the user. It is part of a simple mail transfer protocol (SMTP) that creates email messages and sends them to a mail transfer agent, which will, in turn, make sure it gets to the recipient. This is what users interact with rather than a mail server such as MTA which transports email.
MUA can be web platforms/apps such as Yahoo or Gmail, or they can be software applications such as Microsoft Outlook. They provide numerous services for email users, including message engagement, composition, and reception.
How Does a Mail User Agent Work?
If you want to send an email message to somebody, you will use a mail user agent of your choice to write the message and send it. The MUA you use will contact the simple mail transfer protocol (SMTP) server, it gives information to the SMTP about who is sending the email and to whom it is addressed.
Next, the MUA will send the email header and the message content, when the SMTP receives the message and agrees to deliver it to its final destination the MUA will inform you that the transmission was successful. So the email has been sent.
However, the SMTP stores the message and tries to deliver it to the recipient using an MTA, If it is just one server involved, the message will be transmitted directly to the recipient but if the person is very far from the origin of the message and also uses a different email provider then there will be many intermediate servers before it gets to its final destination.
So, the SMTP of your email provider uses a DNS to detect which SMPT server manages the mail for the recipient’s domain, then your SMPT server connects to the other SMPT server and transmits the message, this transmission continues until it gets to the final server and then the message will be stored until the recipient retrieves the email using their own MUA.