At the moment,the only way is to change the value in the code and re-build postler.
So you could download postler from the git repository,change the value 600 in line 293 in postler-service.vala to what you need and then build and install postler again from source..
At the moment,the only way is to change the value in the code and re-build postler. service. vala to what you need and then build and install postler again from source..
So you could download postler from the git repository,change the value 600 in line 293 in postler-