I am more than happy to subscribe to Mailspring and actually think it exceeds the majority of the other products on the market (ease of use, UI/UX, features) but it looks like development has stalled and with basics like certs expired / no Outlook365 support etc, then how can anyone be expected to commit to this product and pay for it?
I get it is a vicious circle but PLEASE lets rally behind Mailspring and push the product forward as there is something special here 
I’m going to be honest; I wouldn’t subscribe to Mailspring pro unless development picks up. I should caveat it does support Office/Microsoft 365 Outlook/Exchange…if your provider/tenant enables IMAP and SMTP. Otherwise yes, it doesn’t work. If you need that support, I will look into other mail clients if you need Exchange native support. Thunderbird’s latest release has early support for Exchange EWS.
TLDR; No don’t subscribe to Mailspring Pro. Product isn’t really maintained. Not sure what is going on with @bengotow. Probably things going on with life and/or work.