Then it would appear that the software should be changed, so that after your FIRST attempt to post within 300 seconds it then resets the clock to 300 every time you post.......then you won't get some idiot blocking the server by making at least seven stupid attempts to post a message when they know it will be rejected.
I was trying to WORK on here this morning, and sometimes got the message "Server too busy. Try later" - possibly caused by somebody with a failed sense of humour trying to post seven times within the 300 seconds.
Maybe, just maybe, the 300 second rule has been generated by posters like you!!