You may be familiar with some error codes such as
421 - often means: too many users logged to the same account.
530 - wrong login:pass, some servers auto-switch to 530 from
421 when they reach the max # of users. so notice the error message attached to the code.
550 - common in Ratio site, If the file exsist it means you have no access to the file or dir.
etc...
However, do you also familiar with them?
110,120,125,150,200,202,211,212,213,214,215,220,221,225, ect?...
lols