I don’t know anything about iPads really, but I do know a bit about email.
You can’t just randomly send email through any old email server. This is a security feature – it’s too easy to launder spam otherwise. So one of two things needs to happen.
Either, you need to use the ISP’s email server, or if you’re using your mail provider’s server then you need to authenticate. So for example, if your email provider is Gmail and your ISP is BT, then typically you’d use Gmail’s server for inbound email (because that’s where your mail is stored) but you’d use BT’s server for outbound mail. If instead you want to use Gmail’s server, then you’d need to tick the ‘requires authentication’ box and supply a username / password.
An added complication is that a lot of providers now require a secure connection. You need to look up the SMTP settings / requirements for whichever provider you’re using.
Or just use webmail. I can’t remember the last time I used a fat client outside of work.