Examples of legitimate reasons include:
the constituent has asked the MSP to take inappropriate action
the case would lead to a conflict of interest with the MSP’s existing casework
the case is contrary to the MSP’s political beliefs
Section 8 of the Code of Conduct also lists rules:
MSPs must follow when engaging with constituents
for how MSPs can describe their roles as a constituency or regional member
for how MSPs can describe the area they represent
MSPs are responsible for running their own offices and there are no rules about how they must do this or how they approach constituency casework.