An unvowelled mim has sukun rather than fathah, kasrah, or dammah. Its treatment depends on the next pronounced letter: idgham shafawi, ikhfa shafawi, or izhar shafawi. The adjective shafawi means labial, reflecting the role of the lips in mim's articulation.
Still mim + م: labial assimilation.
Still mim + ب: labial concealment.
Still mim + any other letter: labial clear articulation.
In «أَنْعَمْتَ», mim is followed by ta and remains clear. In «إِلَيْكُمْ مُرْسَلُونَ», a following mim causes assimilation into one doubled mim. Apply the rule to actual pronunciation rather than to the distance between written symbols.