The short answer is - 'Do not do this'.

Without touching on the marketing aspects of this, the web mail services, all of them, are THE most difficult to get to accept any email. If there is any slight 'odour' about the email, it will be rejected, deleted or sent to the spam bin.

Two immediate problems are that you are using sendmail and have 'must send from known domain' set to false.
This will very likely ensure that all your mail is sent by 'nobody@' or 'www-data@', the Apache web server user, rather than [email protected]. Just that is enough to get it binned immediately. try sendmail -f with the above known domain set to true, might help.

Next, if the MX records, reverse DNS, SPF records and other mail setup are not implemented, or not implemented correctly - instant delete or spam bin.

If you look at your setup, the Email (sent from) and email(contact us), and the additional emails destination, you are essentially sending email to yourself.

Is it really reasonable to expect Yahoo to accept mail with a return path of nobody@... supposedly from you to you? Sounds like spam to me.

Any potential customers using any of the web based services will of course have similar problems, so it's a good idea to sort it out.

Run your domain through www.dnsstuff.com and do a DNS report, and see what it has to say on the mail side.