We receive a 413 entity too large error when sending a file less than the 25mb limit
If you are sending using RAW MIME, attachments are base64 encoded, they end up being about 137% their original size. This means that even messages that are 20MB actually turns into about 27.4MB in the MIME, putting it over the 25MB total limit.
A solution if sending using RAW is to impose a 15MB limit, this inflates to about 20.6MB in the raw MIME, leaving around 4 and a half MB for text content and headers.
Another solution is to not send a message RAW mime message by removing the following from the header.