Every e-mail service has some limits regarding the file uploads and in most cases it's related to how much you can upload and send using the attachments features. By my understanding, you are uploading files that exceed the maximum file size permitted for all attachments. You should resize the pictures or simply send multiple emails with a few pictures only. If you don't like the e-mail service, you can always switch to a more powerful one which will allow you to send bigger attachments regarding file size.
