Starfield SSL not trusted by iOS 14

Since our domain has been migrated to TSO host I have been unable to send emails on iPhone (iOS 14.7.1) from our @[our-domain-name] email accounts.

I am able to receive on iPhone and to receive and send on Mac (OS 10.15.7)

On Stack Overflow I found this question:

We're using the Starfield Root Certificate Authority - G2 SSL certificate and it appears not to be trusted since iOS 14.6 - neither in the app (API calls get rejected) nor in Safari. Does anyone know what happened? I can see this root certificate is still on the list of trusted certificates by Apple and it works on earlier iOS versions.

And this answer:

Check the used certificate. Apple has introduced restrictions regarding the maximum validity period of a certificate (max 1 year). If the validity period is too large Apple considers the certificate as invalid.

How can this be corrected? Or is it a matter of awaiting Apple to fix this in an iOS update?