Mail on Iphone says the connection to the server failed?

I'm hoping someone can help me with this, but my yahoo mail account I have set up on my Iphone has never given me a problem. Then, yesterday my Iphone 4 had an upgrade so I upgraded it. Ever since then every time I try to check my email a message comes up saying "the connection to the server failed" I have tried turning off my phone and restarting it, but that doesn't help, and I tried deleting my account and adding it back, but nothing works. It does however work when connected to the 3g, but not the Wi-Fi. If you can help me Thank you so much.

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