Just for some more information about WHY it works like this: In a world of increasing spam, email services and hosts have a lot of "demands" for the incoming email. One factor is the FROM address, which incoming mail servers use to help diagnose the legibility of the email ... If the FROM field can be populated by any address from the website-visitor, they could use "bill.gates@ microsoft.com" ... This is obviously
SPOOFING, and would cause your server to get a "bad reputation". If your FROM field is populated by a real email, that is authorized to send, and normally on the same domain, then this is a good hint that the sent email is not spoofed ...
Most servers now REQUIRES a FROM field for email being sent, simply because lack of adding the field could lead to bad reputation for the server / smtp.
An email being sent is sent from your server, and not from the visitor who just inputs their email. The server needs to comply with modern email-sending policies to avoid getting a bad reputation, and to avoid emails being tagged as spam. To do this, the email being sent should have a verified sender (from) that is allowed to send on behalf of the website ... It can't just "send emails" without following modern policies and requirements.
To make things more complicated ( :/ ), one should
really use
SPF and
DKIM to clearly verify to that your server is authorized to send on behalf of the FROM email. This implementation will basically tell all email hosts (google, hotmail, server-based spam-filters): "Hey I am an email and you can be 100% sure I am not spam because I have SPF and DKIM records in my domain DNS explaining that this sending-server is allowed to send for this email@domain!" ... It's basically the ultimate key to avoid emails being tagged as spam
Sender Policy Framework (SPF) is a simple email-validation system designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain comes from a host authorized by that domain's administrators.
At Imagevue and Flamepix, we use
Mandrill email service, which helps setup a proper framework for sending, and makes it simpler to diagnose. We have added SPF/DKIM records to our www.photo.gallery DNS, and use an authorized valid email for sender (from):

... which gives us an excellent sending reputation:

Any incoming email hosts (including Google/hotmail) will see our emails as a VIP "hey, let this guy through!".
So, even if you don't setup SPF or DKIM, it is important to understand that the FROM field should be valid, as it is used to verify the authenticity of the incoming email. If it was set to the person who visited the page, then your server is technically "spoofing" emails that it is not allowed to send from.
When we receive emails, it may look like this:
To: me,
From: me (server),
Reply-to: the guy who sent the email. It may sound strange, but there is nothing wrong with this verified setup.
SMTP
Normally, if you use an SMTP service like gmail, I believe the FROM field will automatically be forced to the same as the account email ... which makes sense.