mail

Ok, who can help me on this.
Tried to send a few photos earlier today, then I got this pop up saying that the size was too big and try later.
Since then it is always loading, the little round loading symbol (not the beach ball) doesn't stop.
Tried a few things but its still doing it.
I searched and found something about deleting off line cache ..... but that means nothing to me, can anyone help me with this but in normal language.
Its a 2009 macbook pro.
Cheers