Don’t attach a huge amount of files to an email message. Not only do people consider you to be lacking in common sense, it may also cause hang-ups within the email system.
Many mailer applications have an upper limit of attachments. For example, attachments over 25 MB can’t be sent from Gmail.
Contact Form 7’s config-validator calculates the total size of attachment files (in the case that files of the allowed maximum size are uploaded), and if the size is over 25 MB it will warn you with this message.