Like us, many of you have multiple email accounts. Yes, it would be cool if we could associate a signature with a particular email account.
Unfortunately, NO Third Party application on the iPhone has the ability to select a specific email account. Whenever Signature launches your Mail Application, it ALWAYS sets the email account to the default one. We agree, this is annoying!
To select another email account when you are in the email compose screen, tap on the From: field and it will allow you to choose a different email account to use to send the email.
We can only surmise the inability to select a particular email account is for security reasons.