When it comes to emails, one of the features I learned early on to appreciate is to not give out your "core" email address to everyone you come across. Yet at the same time you need to share your email address in order to be able to keep in contact with people, that's the whole point of having an email address, right?
So just as some people have a P.O. Box or/and others use call forwarding from one mobile number to another to retain their privacy yet be "accessible" to individuals they might not know so well, there are plenty of reasons someone might want to do the same with emails, one such serious security reason would be SPAM! Another common reason would be the need or want to be able to use one inbox for both personal and business emails yet reply to them as you were doing so from different accounts.
For the past decade or so I have been using email forwarding, that means I would have an email address that I give out to people and when they email me it would automatically and instantly be forwarded to my mobile device(s) and to a number of other email addresses (different servers out there), and when I would reply to these emails it would look like the email was coming from that very email address this was done in a very straight forward fashion in Windows based Outlook like this, and fortunately for me the Danger powered Tmobile Sidekick devices always was able to do this with up to four separate email accounts.
Now with the growing popularity of Android devices and thereby Gmail being the core email system for these devices, I started looking into the possibilities to do this with Gmail, and yes indeed Gmail has no problem allowing you to do this, there is very nice instructions in the Gmail help and support section for how to setup the alternative or custom FROM email address here and once you have that setup you would most likely also want to add the same custom email address to the reply-to part as shown here. The reply-to part is what tells what address the receivers email client will put in the "to" section when he/she clicks the reply button to reply to your message. Now while it will not be visible on the mobile versions of Gmail once you have this setup with custom email address(es) and confirmed them, Gmail will automatically insert the "reply-to" but make sure you have the "Reply from the same address the message was sent to" marked below your custom email address(es).
Word of caution, while these features can be very beneficial for reasons mentioned above, these features are also utilized by preying individuals out there that might be out to use Phishing to get your personal information. So due to this sometimes using this feature might end up with the result that your email might be marked as spam by the receiver but this is generally speaking not too common. Also understanding how these things work or can be made to work, helps one to understand why it is always important to keep one eye on if the reply-to email address is the same as your email inbox shows the email as having come from.
I know this became a bit long on a very simple thing, but hopefully someone will find this useful.
