File Upload Problem

I have created a simple contact form for my website for visitors to upload the necessary information and files, with the following short code for file upload (which is not a required item):

[multifile multifile-437 limit:20971520 filetypes:jpg|jpeg|pdf]

Things worked out alright except when there was a file upload. The form could only be submitted successfully with a file size (or total files size) of about 750K or less. Above this file size there would be an error that ” Sender’s message failed to send “.

I had tested by sending an email to my website attached with a pdf file of 7.7M, and another test with 3 jpg photos of about 5M each. Both tests were successful as the web server delivered these two emails to my personal email address. So email service should not be a problem.

Would appreciate if anybody could give me some advice on this.