Mailbox Service account

That's probably not the most secure tactic. It would be best to instruct your users to log in to OWA to reset the password on their accounts if they expire and they can't get to a domain joined computer. If you set things up the way you want to, any user with an iPhone could access anyone else's mailbox with no difficulty at all.

That said, you can create a generic account with a mailbox then grant permission for that account to access mailboxes and send emails. The commands to do that in the Exchange shell are as follows:
For Full access:

get-mailbox | add-mailboxpermission -user <service account name> -accessrights fullaccess

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For send as:

get-mailbox | add-adpermission -user <service account> -extendedrights "sendas"

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I have tried those both but the email still does not load on the iPhone? When I send an email it arrives as sent from the specified service account and not the user the service account is authenticating to.  Please remember this is for an activesync deployment.

If your company is a public company that comes under Sarbanes Oxley or a health care provider under HIPAA, this would be a very bad thing to do with legal repercussions.   It would amount to a gaping security hole and let any savvy user, tech support person, previous IT person, that has the iPhone account password read anyone's email.  They could use that account to read email even using OWA.  I'm assuming you would never or hardly ever change this password.  What if someone outside the company got a hold of this information?  User education as to the importance of changing the password, even on the iphone is the best way to go.

Because activesync allows your email to go out to the Internet, this is about the same as not requiring users to ever change a password.  Think about this.  Your internal network is much more secure than the Internet.  If your management would go along with having a non-expiring password account to access email over the Internet, they would probably think it is OK to not change passwords on their regular accounts.  

But a never changing password is just bad, bad, security practice.  Even if your boss is asking you to do this, I would protest and explain.

There's a special setup required to do this on an iPhone. The whole setup has to be manual and can't use Autodiscover. And each time the user sends an email they have to explicitly change the "from" address. I don't have an iPhone near me to test this with or give you instructions, but you are seriously better off not using this method. Just instruct your users to log in to Outlook Web Access and change their password from there when it expires. They will be prompted to do so automatically as long as Password Change is enabled, which can be done from the Exchange server easily enough, and is the default setting.

I see your points but this account will be pushed out in configuration profile and no one will ever see the account credentials but myself.

I hear all of you and I will accept it as the answer.  I will not pursue this adventure anymore, case closed.  Thanks for all the comments!