I'm curious why e-mails addressed to "undisclosed@abc.net" reach my inbox. Undisclosed is really the address.?

Also, e-mails addressed to a name other than mine, but with the same domain reach my inbox. Example: my e-mail is "myname@abc.net" but I get e-mails addressed to "anothername@abc.net". I also received an e-mail lately that I thought was curious even though it was from a known and trusted sender. I will try to describe the header without giving away any specifics. The first line said "Fw: Subject of E-Mail". From was "Senders Name" . To was "Me" where "Me" is an actual, direct quote from the e-mail. Also, sometimes I click on an e-mail in my inbox and an e-mail in another folder opens. Please, no answers that don't understand why I'm not revealing actual addresses or otherwise can't comprehend the question.