In a similar vein to what Jofm5 describes, I exploit this "from" feature to my advantage too; I prefer to use a mail client (in my case currently Mozilla Thunderbird and before that Outlook Express) installed on PC in lieu of the web mail access provided by all three mail service providers that I use. In Thunderbird I have therefore defined three mail separate accounts, each for a different address and each associated with a different domain. Only one is associated with BT, my internet connection provider.
I quite happily send mail "from" all three domains using the BT connection as well as receive (download) mail to them. BT know who I am in terms of authenticating the actual "send and receive" command (it will only work from home, on my own connection, not anywhere else where I take my laptop), so it succeeds. When I first discovered I could do this, for fun I invented a totally silly mail address name and defined an account on the PC using it as the "from" address. Something like
[email protected]
Of course it sent without problem (as it happens to my work address where the spam filter successfully trapped it).
For the record DB6, my work spam filter traps fifty mails on a typical day of which 49 are probably using my own work address as the "from". There is no way on earth that anyone other than my employer's mail administrator or me has access to my account.