Spam.

I'd stop putting your email address in your .sig; they're using bots to search the Web looking for linked email addresses and you're essentially inviting spam.

Members can PM you. They don't need an email address in your .sig.

You've got your email in your sig, and posts are public (and even indexed by Google, which is convenient when I want to know what I or someone else said a while back), so they're going to scrape your address and do that.

I'm guessing this is a low-volume type thing (as in they send tens of thousands instead of the usual hundreds of millions of spam most high-volume spammers you never see send) that tends to fly under the radar of the major ISP's and spamfiltering services, but it can't hurt to forward the spam to them.

I Googled the company name and got many hits, perhaaps most interesting is the SEO Marketing Spam Hall of Shame. It looks like this scammer only made #20 of 30 such scammers.

Take out your email from your signature. If you must put your email do it like rowanlindsay[@]rowanlindsay.co.nz or something to disguise it from bots and people trawling for emails.