Again, i don't think your iOS mail client is correctly configured, seems SMTP authentication is missing.
With SMTP authentication, postfix will bypass the HELO check completely, so no "Helo command rejected" error at all.
I don't know any other way to configure iOS Mail App, I selected Exchange, and then filled in the required fields. I work with a lot of native exchange and Office 365 deployments professionally. There is no option to configure SMTP anything for an exchange active sync mail profile.
It is my understanding the SoGO is used to facilitate the EAS connectivity in iRedMail. It seems like from the logs that when the connection comes from SoGo it's not trying to use TLS, is there a way to check that, or some place in SoGo logs that I should be looking?