Blacklisted IP Addresses
We have experienced a few incidences recently where outgoing emails are being rejected by the security system on the server because the IP address you are sending from has been blacklisted as a spam sending address.
IP addresses are usually assigned to you dynamically by your Internet Service Provider (BT, Talk Talk, etc). Theoretically this means that each time you connect to the internet you are assigned any one address from a pool of addresses available (with a router this is generally re-assigned each time the router is turned of and on again). If a previous user of an IP address has been carrying out activities which lead to that IP address being blacklisted then that blacklisted IP address can be passed on to quite innocent users causing problems for them.
If this is the case with the problems you are experiencing you will receive an error message saying that the IP address you are sending from is listed in the Fortiguard Antispam database. Your first course of action will be to go to the Fortiguard website and request de-listing of the IP address you have been assigned which will be shown at the bottom of the rejection email you receive. Secondly, contact your Internet Service Provider and get them to ensure your IP address is de-listed, but do not let them talk you into changing any of the settings on your system, if the IP address shows on the Fortiguard website as blacklisted then it is a problem with their IP address they have assigned to you, not your settings.
You could also try turning off your router, waiting a minimum of 10 seconds and then turning back on again to see if you get a different IP address assigned to you.