It's a very polite expression, saying in easy word, "Glad to see you", I soppose. It's so difficult to traslate English to Japanese directly. (sorry my English isn't so good)
縁 means relationship or chance between peoples, plaeces, and everything. In this case it says the chance of meeting / knowing each other, the chance of between you and the person who sent that mail.
And we use ご before nouns to express courtesy, sometimes we use お, depends on the nouns.
Hope it helps you.