TL;DR: The size limit is currently configured at 30 megabytes.
This is the maximum size a message may be when arriving at our email servers.
Please note that this not only includes the message body and attachments, but also the headers that a mail client and other server infrastructure adds. Usually, these amount to a only few kilobytes at most.
Furthermore, more headers are added within the Kolab Now infrastructure — while the message is being delivered to your mailbox, these additional headers could, potentially, cause the message size limit to be exceeded.