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.
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.
*This limit applies to the size of the entire message and not just the size of the attachment.