The domains are hosted on my server. Does this mean that I need to install a email server?

>The host should be able to provide this service.
>
>>Hi
>>We are hosting a number of web sites for customers. They are using their own ISP for dialup access.
>>For example:
>>Web site address www.cyberprop.com
>>Their current email: ultimate@netactive.co.za
>>They now want to receive email with their web site address for instance willie@cyberprop.com
>>
>>The problem however we are finding is that the ISP do not want to give them the email address (willie@cyberprop.com) if the domain is not hosted by them.
>>
>>First price for me:
>>A solution where I can relay their email from willie@cyberprop.co.za to ultimate@netactive.co.za
>>
>>Does anybody know of a product that will be able to solve my problem ??
>>
>>Thanks
>>
>>Willie van Schalkwyk
>>www.cyberprop.com