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It's easy to forward one email address to your real email account, but sending email to look like it came from that forwarded address isn't as obvious.
How do I send mail from my mail forwarded address?
There are services that automatically forward mail from one email address to another. Registering your own internet domain is a common example. Most registrars will offer to forward email sent to that domain to an email address you specify.
Say you register "example.com" and email sent to example.com is forwarded to your real email account. What if you want to send mail that looks like it came from example.com? In most cases it's not difficult, but exactly how depends on your mail program and perhaps even your ISP.
For the sake of discussion let's define a couple of examples:
email@example.com is your "real" email address. Obviously you have a real email address that is not actually "firstname.lastname@example.org", but we'll use this example in this discussion. You get your email from yourisp.com and everyone knows to email you at "email@example.com". Email you send is marked as "From:" firstname.lastname@example.org.
example.com is the domain you've just registered, and/or email@example.com is the email address that is being automatically forwarded. Any email sent to "firstname.lastname@example.org" is automatically forwarded to "email@example.com".
The problem is that when you reply to email, regardless of whether it came directly or via the forward your reply is from "firstname.lastname@example.org". The "trick" to making it come from "email@example.com" instead is to set up a second send-only email account in your email program. (If you always want your email to be sent from "firstname.lastname@example.org" you can instead simply modify your existing account settings as I'm about to describe.)
Exactly how and where you'll make these settings will vary based on your specific email program. There's a small chance that your program doesn't support all of these settings though most do. Web mail services such as Hotmail, Yahoo, and the like typically do not.
The new account is exactly the same as the account you already have for "email@example.com", with the following changes:
The email name is "firstname.lastname@example.org". Important: the email name is not the same as the account name. The account name is how your ISP knows that you are you. We'll still be using your ISP to send email even with the different "From:" address. Though your email program may have a different setup, here's an example from Outlook 2003:
The account should not download email unless it's to be your only account. Your email arrives via your regular account only.
Depending on your ISP, you may need to select outgoing authentication on send. Options include specifying your ISP's account and password again, or possibly somethign called "POP before SMTP", meaning that your ISP will authenticate your sends if you first download email (with your real account).
Caution: this is the step that may cause your ISP to reject sending using your custom email address. Some ISPs in the interest of thwarting spammers insist that email you send must be "From:" the email address that the ISP assigned you. Fortunately those ISPs are becoming fewer and fewer, but unfortunately if your ISP enforces this restriction there's very little to be done. You'll need to use another service to send your email.
Once you've defined that email account, you'll need to tell your email program to use it when you send mail. When you reply to a message, or when you compose a new one, your email program should allow you to select which account to send as; it will probably default to your regular account. Select this new one, and you'll send as "email@example.com", meaning that's who the email will appear to come from.
If you're concerned that this technique might somehow be used to hide your identity or pretend to be someone you're not remember that the the mail header information that you normally don't see will still contain information about where the mail was sent from.
Here's a video walking through the changes described above using Outlook 2003 as an example.