Unfortunately even if Fly offered (as Azure/AWS/etc allow for their customer’s IPs, by support request) PTR records for your app’s public IP I don’t believe the PTR requirement would satisfy a PTR-for-sending-SMTP-from-Fly use case.
Any outbound SMTP, or indeed any TCP, connections from your app(s) will not be sourced from the app’s Fly public IP, see also here. Nor can you currently define the originating (shared) IP. Even if Fly offered PTR records for Fly app IPs - the outbound-static-IP issue would likely be a problem (re: SMTP/PTRs), as they’re not going to allow a single customer to set the PTR records for their numerous IPs from which your app’s traffic could be originated from.