While it is true that 2132763 is less than 12582912, what the message is saying is that "I tried to allocate X bytes of memory, but that would have exceeded the maximum of Y." Even if X < Y, the application has already allocated a bunch of memory before that line of execution, so amount used already + X > Y which is the problem. Try increasing the memory limit for that virtual host, or the roundcubemail directory in particular.