Help WiFi & 3G emails need different smtp server?

My broadband WiFi uses Talktalk. My mobile phone service provider is Giffgaff.

When I use my phone to send an email while at home within range of my WiFi I have to set my SMTP server to talktalk, not a problem. But when away from home and using 3G to send emails I have to alter the SMTP to o2 (giffgaff uses the o2 network)

I tried setting up two accounts with different servers but my phone tells me that "email address already exists" and wont let me.

Am I missing something obvious? Is there a way to avoid going into settings to keep changing the server?