Spamming and the associated issues related with it are somewhat complex, but we can start with a very clear known, that unless you are running Windows on your mini, the issue is not related to a virus! The reason is that while there are many, many variants of malware as a whole that can result in harvesting email addresses and the creation and mailing of email messages without the user's knowledge or consent, there are none that are so far reported to affect systems running MacOS - and believe me, with the money security vendors are happy to make selling their wares to MacOS customers, we would know for sure when the first glimmer of such things appear.
That isn't to say it won't happen at some point, but that with the rapid escalation of malware in recent months being heavily biased towards money-generation in one form or another, it isn't a particularly worthwhile and productive notion to develop an effective malware package to attack the Mac platform - at least not at the [present time.
However, it is perfectly possible for a Spammer to harvest an address book which includes your address from a friend or family member's Windows system, or a web site or service that gathers email addresses of clients/customers and to which you and some of your family and friends have signed up to - or indeed any similar internet resource - and end up spoofing your email address (invariably amongst many others) and using it to spam others, including those from the same address book.
If you do run Windows on your mini and don't have similarly effective security software that would be expected in any other Windows system, then you are vulnerable, but otherwise really not..... yet at least!
