Email spoofing is when messages are sent out that appearing to be from a different address than they are from.
I encountered an email spoofing attack this weekend when emails were sent out from an address I have not used in several years. The addresses it was sent to came from some lists from a time back. The message was titled I lost 2 sizes in a week. The message bounced on several addresses and come back to me which when I discovered it.
The message was not sent from the account it claimed (I checked the account and it had not been entered in over a year (about last time I glanced to see if it was still live) and there were no messages sent from it. The contact list there is only two names and those were not the addresses that bounced.
Someone had sent an email from their account and made it look like from me (spoofing) and had gotten some messages with that address in and sent to them. The message was a link to an account in Belize and probably was either a spammer collecting addresses or someone getting passwords, etc off people and machines who go to it.
There is really no way to prevent your account being spoofed. In the regular mail system (USPS) it is possible to spoof where mail comes from the same way and no way to know where really or to stop. We could send out letters with your name on them and your return address on the envelopes and it will appear to be from you. This is same here.
There are a couple things you can do to reduce the risk of spoofing. First when you forward emails, clear the list of addresses of the message. Spammers love to harvest these as very likely current live addresses. Second when you send a message to a number of people don’t list them in the To or CC options (they will appear on everyone’s message) but list them on the BCC (blind carbon copy) and the recipient will only see the To address and their address. The rest are hidden. These do not guarantee your address will not be spoofed (I do both of them and have for many years) but reduces the chances.