I hope this helps someone out including me when I forget about this and need to come back here
Basically Google'ing around I came across this thread:
Same problem, same issue deeper pockets (by that I mean better machines and network then me!)
Solution is linked in but just to save time and effort can be read here:
Zimbra - Forums - View Single Post - Max imap message size
Really all we are looking to do is run this:
50x10^6 is the size presumably in bytes.
zmprov mcf zimbraFileUploadMaxSize 50000000
And just to note that after issuing this you will need to restart Zimbra;
I found out the hard way as I think the GUI makes changes instantly while the CLI updates config files which are read on the next boot. If anyone else has better insight on this the knowledge would be appreciated!
That's all for now though - have fun folks.