cannot send email since uploading el capitan

Please help I have only had my macbook pro with retina display for 2 weeks and it is my first one, although I do have an iPhone and iPad.

It's been working great until last night I uploaded el capitan and now I cannot send emails. I am receiving them but cannot send any.

The message i get says "Cannot send message using the server sky" or when I try iCloud I get the same message but says "iCloud" but I don't know how to do this. I am a novice really so any help would be very much appreciated.

Do I need to contact my internet provider ?

Many thanks