1. "Connection failed" error.
2. How to configure a Gmail account?
3. How to configure a Hotmail account?
4. How to configure a Yahoo account?
5. Passwords aren't saved.
6. Something isn't working, what should I do?

1. I receive "Connection failed" error. Other mail clients work fine with the same settings.
Check your Firefox network settings. If you have a proxy specified and "Use this proxy server for all protocols" checkbox checked, then your proxy is used for all protocols. But are you sure that your proxy supports them? Simple Mail makes use of SOCKS proxy settings. Most likely your proxy doesn't support SOCKS, so uncheck "Use this proxy server for all protocols" checkbox to solve the problem.

If you use a Firewall, think of adding rules to allow Firefox to use ports reserved for e-mails (25,110,993, 465, etc),

2. How to configure a Gmail account?
Below are links to configuration instructions for IMAP and POP3 protocols. For SMTP use port 465.

Gmail through IMAP
Gmail through POP3

3. How to configure a Hotmail account?
You have to use these settings :

POP3 Server : pop3.live.com with SSL/TLS
or
IMAP server : imap3.live.com SSL/TLS
SMTP Server : smtp.live.com with STARTTTLS

login : yourname@hotmail.com

4. How to configure a Yahoo account?
You have to use these settings :

POP3 Server : pop.mail.yahoo.com. You will have to enable “SSL” and use 995 for Port.
SMTP Server : smtp.mail.yahoo.com. You will also have to make sure that “SSL” is enabled and use 465 for port.

You will have to enable “Allow your Yahoo Mail to be POPed” under “POP and Forwarding”, to send and receive Yahoo mails through any other email client. This is done under the Yahoo mail interface, under "options" then "More options"

See also :

5. Every time I restart Firefox, I have to re-enter passwords for accounts in Simple Mail.
The cause of this is that you have "Remove saved passwords on exit" option specified in Firefox options. To change it, do the following: open Firefox menu "Tools > Options", select "Privacy" tab, look at "Private Data" section, click "Settings..." button and uncheck "Saved Passwords" checkbox in a window that appears. You may also want to uncheck "Remember passwords for sites" on the "Security" tab.

6. Something isn't working, what should I do?
Please collect information about error messages:

• Enter "about:config" in the Firefox address bar. A page with a "Filter" input field and a list of preferences appears. Enter "javascript.options.showInConsole" into the "Filter" field, single line will be displayed. If column "Value" contains "false" then right-click the line and choose "Toggle" from the context menu, so that value becomes "true".
• Open Firefox Tools > Error console menu. An error console window appears. Click "Clear" button there.
• Now do what you were doing to reproduce the error.
• Go back to Error console window and see if there're new messages.

You can also, since V2.85 enable log file

Send these messages along with the problem description to the author through forum email. This will help to solve the problem.