Must be the time of year cos I keep getting things sent back(postmaster failed delivery) that I haven't sent.
Thats because a spammer is using your email address as the reply address. I have seen businesses bought to their knees because of this and there is very little you can do...
You just have to be very selective with the emails. It's easier with a private mailbox but commercial mailboxes are very difficult.
Private boxes: just don't download any where you don't know the sender, but spammers will get by. Don't use a POP mailbox (they download the whole email to your computer), use an IMAP mailbox as these only download the header (provided you don't click on the email to download).
Commercial boxes: it's very difficult to know which are genuine and which are spam, it needs an operator who is very knowledgeable about your business to pick out the spammers, don't leave it to someone on "work experience".
I used to run several websites and as such I had to download ALL emails, I got no end of spam.
There's no easy way to deal with spam.
