Could it be possible to have trans fluid leak from the rear main output seal are if the trans fluid level is too high? Doesn't seem intuitive. My guess it's most likely a bad seal or perhaps it needs some RTV between the seal and surface?
I just did a trans swap (replaced the seals on the trans as well), and I was trying to fill up the trans with fluid. Well I couldn't seem to get it to fill up/drip from the fill hole...and after some time pumping a small leak developed near the rear output shaft

 

What kinda seal did you use? The parts-store ones all seem to be junk. But you say "near the output shaft", is it AT the seal? Or near the seal. Identifying where it is actually leaking is a crucial first step.

 

I used a national rear output shaft seal. Part # 710818. Is that a decent brand? I have a SKF seal now to replace it, if I need to. It's leaking at the rear main output shaft seal.

 

If it were me I’d only use a genuine OEM Mazda seal. Every part store brand seal I’ve tried over the years either leaks right away, or like 3-4 weeks after the jobs done. The legit OEM stuff actually works. They’re usually not even that much more expensive

 

From what I've heard National and SKF make quality seals, but probably not as good as OEM. I swapped the seal out with an OEM one and it's no longer leaking. Thanks

 

Coulda fooled me lol. Glad it worked out.