A user can receive email but cannot send any. Which protocol, responsible for sending email, is potentially not configured properly?

A user can receive email but cannot send any. Which protocol, responsible for sending email, is potentially not configured properly?

• SNMP
• FTP
• POP3
• SMTP

EXPLANATION

The simple mail transfer protocol (SMTP) is probably not configured properly.
When troubleshooting, you need to confirm the SMTP setting because it is responsible for sending mail. POP3 receives mail, so wouldn't be responsible for the send issue, unless you are using Outlook (where POP3 authentication is required before email can be sent over SMTP). FTP sends files to remote computers and SNMP is used to manage networks.
FTP and SNMP are not responsible for users sending or receiving email.

SOURCE

http://searchexchange.techtarget.com/definition/SMTP