10 MB or less is generally considered a good size limit for email servers.   Increasing the limit beyond this can make the server unreliable or vulnerable to attacks for the reasons listed below. 

In many cases, because your email client (a program such as Outlook) does not communicate with the server about the size of the file, the whole email including the attachment will be sent, which can take a long time, and then the server will ‘bounce’ the email, sending the whole email back to you along with a message saying the email is too big to send! 

To avoid this, email clients should be configured with the appropriate attachment size limit, so if the server has a maximum message size of 10 MB, set the client to have a maximum attachment size of 7 MB.

How to overcome the limit? One option is to make a zip file, split this into multiple 7MB parts and send each part in a separate email.  However, this is inconvenient, and the virus scanners on some email servers will reject .zip files anyway. At the end of the day, the email system is just not designed to handle large files.  The best way to send large files is to use a Managed File Transfer Service such as LeapFILE.  It allows you to upload your file to a server, and then the person receiving the file downloads it from a website.  The system is secure, faster and more reliable than email.