IP addresses do get black listed from time to time and can be easy or difficult to get de-listed depending on the cause of the listing and the actions taken by the people involved. If you're with Afrihost then use smtp.afrihost.co.za as your SMTP server. If you're not then please do contact your ISP to find out which smtp server they provide.
Incoming mails could be delayed because the server you are connecting to, is having issues that are causing mails to be delayed; your mail client is not syncing with the server at the right time to receive mails on time; or your ADSL connection could be experiencing slow connectivity because of the ISP or DSL line issues.
You shouldn't be sending out mails using your ADSL IP. If you are on the Afrihost SMTP still having an issue with sending mails you probably have a virus on your computer and you should get rid of it soon. The reason why the IP's are blocked is because some people use their connectivity to spam. One person can send spam on the same IP and the IP will get listed as a source of spam. Another person does the exact same thing that the other person did and now the IP range is marked as the possible source of spam and spam gets blocked at places that consult the list. You on the other hand get the same IP range and try to send out mail that is not spam and you get blocked because of previous incidents. Now any IP that you get, gets blacklisted.