I should note that in Latin the masculine agentive suffix is definitely -or. It may look like -tor, but that -t- is actually a separate element marking the perf

I should note that in Latin the masculine agentive suffix is definitely -or. It may look like -tor, but that -t- is actually a separate element marking the perfect passive participle form. (From which agent nouns are derived.) Most verbs have a passive participial form that includes the -t-, but in some it's been swallowed by an S. The only example that comes to mind is missus [sent], the passive participle of mittere [send]. The modern French verb is apparently mettre. I guess if the agent nouns are formed from that root, they should be metteur and mettrice? But if they descend from Latin, the masculine form might not use -tt-. I feel pretty safe in saying that "sender" would be missor in Latin - it's an attested (though "very rare") word - but I have no instinct for what the feminine form would be. Missrix just doesn't feel right.