In any moodle instance, use the address "/local/codechecker/?path=blocks%2Fquickmail&includewarnings=1" to run the code checker. You'll see many error to fix. (Tried on moodle 3.5 with plugin v1.7.6)
We're moving beyond the 1.x branch of Quickmail and are concentrating our efforts on 2.x. Passing all Moodle code checks is a high priority and will be done.
In any moodle instance, use the address "/local/codechecker/?path=blocks%2Fquickmail&includewarnings=1" to run the code checker. You'll see many error to fix. (Tried on moodle 3.5 with plugin v1.7.6)