Each email application used an email server for sending emails. These email servers are called as SMTP servers free email service supplier companies like Gmail, Yahoo, AOL, Outlook, Hotmail.
You can connect any SMTP server via APIs for sending emails with your local application.

Simple Mail Transfer Protocol (SMTP)


Simple Mail Transfer Protocol (SMTP) may be a text-based protocol for transferring email messages from a mail client to a mail server, and from one mail server to another mail server. SMTP deals with transferring mail from client to server and server to server.

POP3 and IMAP are the protocols that we used to Email clients receiving purpose protocol. 
POP3- Port office protocol version 3 (port 110 and secure port 995) 
IMAP- Internet Messaging Access Protocol (port 143 and secure port 993).


Internet Message Access Protocol (IMAP)


In comparison, IMAP is taken into account because the higher and most suggested choice for checking emails across multiple devices like a laptop computer, desktop, smartphone or featured phone. IMAP ensures that emails are synced across multiple devices.

Post Office Protocol-3 (POP3)


On another side hand, POP3 downloads email from a server to a single device and deletes it from the server. After downloading your email message to a single device then it deleted from the mail server. If you try to check it from a different device it can appear that mail is missing or deleted from your inbox. IMAP is certainly a much better protocol for receiving emails