It is too late.

If your ISP didn't reject the incoming e-mail then whoever sent the test knows the address is valid. Be prepared for more spam.

Any worthwhile organisation sending just "test" are not the kind you should be trusting. They would be idiots.

I had an e-mail from a job agency listing just about everyone on their list. They said it was a mistake - one month later the spam started. I use unique addresses for specific areas of e-mail so when I pointed-out to them it was how spammers get the addresses they basically ignored me. Truth is this was a virus designed to target organisations with large databases of addresses - but they could not bring themselves to admit it. I told them anyway - I was not fooled by their lies. That address is now closed.

I never had e-mails with strange characters until after I looked at my e-mail in Thailand - so there you have another security hole. Internet cafes.