If you can't send e-mails or the option is grayed out
This means you do not have an e-mail account configured in your Mail app
An e-mail account is required to send e-mails. It is Apple's Mail app, it is not Gmail, Outlook, Edison, Spark or any other app.
E-mails are handled by iOS and the Mail app. iDoceo does not send them or has access to any e-mail configuration or data
It is like this by Apple's design, to protect your mail configuration, and data from third party apps.
Default e-mail address
All e-mails are sent by the Mail app in your iPad (that is the iOS system provided Mail app) and it solely depends on your iPad settings. iDoceo has no access to your e-mail configuration, account details or messages.
In recent iOS versions, from Settings > e-mail you will be able to decide the default account from which e-mails will be sent.
Careful, this account must be a valid account from the ones configured in your Mail app. Any other account won't work.
In previous iOS versions
If you'd like to change the default e-mail account used by the Mail app: Tap on your iPad Settings > Mail > Default Account as shown in this image