Because containers names defauts are not FQDNs, #2019 was merged.

In k8s, the hostname can be set using the Pod hostname parameter, but dots are not allowed.

Email's messageID are generated from os.hostname in https://github.com/knadh/smtppool/blob/master/email.go#L691

Overwriting the hostname part of the messageID using an environment variable would be convenient for any case where the hostname could not be an FQDN