AppRiver does have a max file size limit* of 100 Megabytes (MB) which is for incoming and outgoing messages. However, if your server does not accept nor send that size you will need to adjust that on your end. If the recipient's server does not accept this size you will need to contact their server administrator or reduce the file size.
For Example: If your mail server will only accept 50MB messages then our server will attempt to deliver the message to your server which will then reject the message stating it is too big. Unfortunately this will not be something we can fix since the size restriction is set on your server. If the recipient's mail server does not accept messages larger than 50 MB we will attempt to deliver the message to their server at which point their server should generate a bounce back which is then delivered to the sender letting them know the file size is too big for the receiving server. This is also something we are unable to resolve for you since we do not manage the recipient's mail server.
*This limit applies to the size of the entire message and not just the size of the attachment.