Either of these addresses work for me:
dbayer@barnard.edu
bayer@math.columbia.edu
I am on sabbatical. I intend to retire, and I no longer have an assigned office.
I enjoy corresponding by email with former students, and meeting by Zoom. Please be understanding if I don’t reply promptly; I’m prioritizing my research more than I was able to do when I had responsibilities. Somehow, “I’ll answer tomorrow…” can turn into “Can that message really be from last month?!” It’s not you, it’s me.