After more testing, this seems to be a resolver issue in sendmail, since I can specify an IPv6 address directly as a smart host and it works. If it's a config problem, I don't know what that might be, and it doesn't explain the existing install no longer trying to use IPv6 after upgrading to Focal.
After more testing, this seems to be a resolver issue in sendmail, since I can specify an IPv6 address directly as a smart host and it works. If it's a config problem, I don't know what that might be, and it doesn't explain the existing install no longer trying to use IPv6 after upgrading to Focal.