Are you receiving any errors? If so, can you examine the error message for any information contained?
1) If you receive a message back
If you see something along the lines of:
123.456.123.456 is listed in xbl.spamhaus.org (http://www.spamhaus.org/query/bl?ip=123.456.123.456)
This means that our new measure has blocked you from sending email through us as your Broadband IP Address has been listed in a third party Blacklist.
If you are sure you are on a Static (unchanging) IP address from your broadband provider you will have to follow the link in the error message (to spamhaus), and follow the instructions there to get your IP address de-listed. However please bear in mind there could be a virus or similar on your network that is sending out spam and so getting the IP address listed.
If you aren't sure if you are on a static IP address then you are most likely on a dynamic one (this is the case for the majority of broadband connections). In this case you can try plug out your router or modem for about 30 seconds and plug it back in again. This should get you a new IP address. If you are still having the problem then you can send your email out through their SMTP server, and everything will work as normal. You will have to contact your ISP to find out what their SMTP server is called.
You can see a bit more on this here:
Other common reasons are if the message is refused on the receiving side. Typically this is due to your email being marked as spam or that the email address is not correct.
2) If you cannot send email and there is no error messages
Is your email program not able to connect to the SMTP server at all? Please be aware then that some ISPs (Internet Service Provider) block port 25, so you will need to use their SMTP server.
You can test this easily by opening a telnet session:
# Start >> run >> type in cmd and press enter (if on linux or a mac open a terminal instead of this)
# type in telnet mail.yourdomain.tld 25 and press enter
# If using Vista please see here: https://help.blacknight.com/entries/22935043-getting-telnet-to-work-in-windows-vista-7-8
If telnet cannot connect it is highly likely that your ISP is blocking the port. You will have to contact your ISP to find out what their SMTP server is and send email out through this instead of us.
3) If you cannot send email but receive an error message
Are you getting an error saying something like "553 invalid recipient"? The most likely cause for this is if you do not have Password Authentication turned on for sending email. Please do not confuse this with "SPA" or "Secure Password Authentication" which is _not_ supported.
You can see how to enable SMTP authentication under a number of common email clients at: