Email is akin to sending a letter through the Royal Mail (with out the cost). But you can only squeeze so much into a letter, before it becomes a parcel.
The same is true with emails, however in this case we just attach documents, and don't really appreciate how big they are, after all how big is 1MB of Data?
So an email with a 15GB attachement is really like trying to send a 15KG letter, it is never going to get through the Royal Mail, it is going to have to go via Parcel Force, or an alternative courier.
Things aren't quiet as simple as that with email, but you will have two limiting factors, firstly your system maybe set up in such a way that you can only attach a document of a certain size, at least you know this part of it. The other part of the equation is that clients email systems maybe set up to only receive emails up to a certain size. Generally you will probably be ok up to around 5MB over this it isn't going to be certain to be delivered.
You have a couple of options out on the web, you could use dropbox, or yousendit. So you can upload the files, and let your customer/supplier access the documents.