Sendmail: “unqualified hostname unknown; sleeping for retry unqualified hostname

Sendmail hang or is very slow when sending an email.

debian sm-mta[8129]: My unqualified host name (debian) unknown; sleeping for retry

Operating System:
sendmail is searching for a FQDN ( fully qualified domain name ). In our case the host name is “debian” and that is not a FQDN. To resolve this problem change /etc/hosts:
FROM:       localhost       debian

TO:       localhost.localdomain localhost debian       debian

Where debian is your hostname.

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