When a Credit for Income Tax Paid to Another State is present on Form 80-105, line 18, Mississippi requires a copy of the Other State return to be attached as a PDF for electronic filing.
The following diagnostic is generating: e-file: When a Credit for Income Tax Paid to Another State is present on Form 80-105, line 18, Mississippi requires a copy of the other state return to be attached as a PDF for electronic filing (ref. #32480)
I have to admit, this is the most interesting post so far this week. As a side note, did you attach a pdf of the other state return to the Mississippi return?