BELD limits email attachments to 10 MB (attachments in webmail are limited to 6 MB). Considering the millions of spam emails we receive daily, if someone were to send us 10,000,000 emails, each with a 4 GB attachment, our ability to process email at all would be shut down because our servers only have so much disk space. The 10 MB limit represents a compromise that allows for most ordinary uses without leaving us especially vulnerable.
To transfer larger files you can use a variety of methods, including using an FTP client (such as FileZilla) to put your files in your personal web space. If you don’t want to do that, there are a number of free and paid file sharing services on the Internet.