Let me be clear : standard gmail (as in you@gmail.com) will always show the 'on behalf of' in the sender.

In order to get a proper smtp service, you need to sign up with Google Apps and have google host your domains' email.  Then your emails will come from sender@example.com as you wish.

http://www.google.com/a/cpanel/domain... is where you start.  Note that it's hard to find the free version now, most of the sales channel will put you into the paid version.  But the free version still exists, I think it's called standard edition.  But it shouldn't be a big problem to pay the extra if you're running a business.

Yes, there is a smtp relay limit with gmail - you can't use it to spam - but I don't know what that limit is and I haven't found it yet with my site.  It could possibly be on a case-by-case basism or just a standard set amount per day/hour/whatever.  If you're at the point where you're sending out more than the relay limit then I would seriously consider using a proper email distribution service or pay for a dedicated hosted smtp server somewhere where you can send as much as you please.

Why not switch to a real host that allows you to send email?

Jeff