Unusual email problem?

Hi, I have a problem which I'm hoping someone can explain and/or resolve.

I access my email using the O2 snot server, smtp.o2.co.uk which works fine on my iPad and iPhone in the UK. I'm on holiday in the US and it doesn't work to send email when using the free wifi at the house I've rented but does work if I use other wifi such as McDonalds or indeed the 3G network.

Is the wifi connection I'm on somehow blocking my connection to the server? I can browse the net ok and receive mail with no problem, it's just sending it that fails.

Would be interested to learn why this might be happening and any advice would be welcomed!