There is a job offer I have which is significantly lower than market rate to my understanding, and the rest of it is good. I am looking to turn this offer down as I'm in the process with other companies which could be better.

How can I best word my declining of this lowball offer while keeping the door open in the future? (In this scenario they have said this is the maximum of the salary band they have available, as we could only determine seniority as we got through the interview process.)

I note that I don't believe telling them the truth would be a good option as admitting the unspoken quiet truth of "money/your compensation is too low compared to other options in the market" could potentially "close the door" so to speak (i.e., between being underpaid in that position and unemployed I'd happily picking being underpaid there, if that situation ever arose, however I am currently employed, thankfully).