There are any number of reasons that could be.  More information is needed.

Presuming the Reply to Sender link is formatted correctly as mailto:personxyz@... then it should open correctly in either the e-mail client you have set in Windows as the default client or in the webmail you're using if you activate it there.

What's happening?  Any error messages?  Do you have a default e-mail client defined and is that client the same as the one you're reading in?  There are a number of things that need checking.
--

Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042  

[Regarding the Supreme Court refusing to hear the case brought by Texas to overturn the votes certified by 4 states:]  Pleased with the SCOTUS ruling, but also immediately slightly terrified of where this crazy train goes next.  We should know by now there’s a bottomless supply of crazy.

        ~ Brendan Buck, former adviser to Speakers of the House Paul Ryan and John Boehner