If a client can receive mail but cannot send outgoing mail, you probably haven’t configured the SMTP and mail servers correctly in their e-mail application program.
Please note, most ISPs will only allow you to send mail if you are connected locally. You must use the ISP local’s mail server to do the mail sending. You can put a “reply-to” address which will appear in the recipient’s “from” field.
mail
MAIL SERVER – checks incoming mail
usually: mail.[domain].[TLD]
where [domain].[TLD] is your mail domain server
SMPT SERVER – your local post-office
Most ISPs only let you send mail from your local post office. Common SMPT servers are:
VERIZON
EARTHLINK
AOL -> Does not support POP3
APPLE -> does support POP3, allowing both sending and receiving
mail: mail.mac.com
smtp: smtp.mac.com