If you have a Debian machine and emails appear to be coming from hostname
/etc/mailname and set this to your actual FQDN. Another instance where I found it locally is in
/etc/exim4/update-exim4.conf.conf, which is almost certainly not read by my installation of Postfix (which does explicitly refer to
/etc/mailname in one place).
This appeared on a machine running on DigitalOcean, so I presume “d” refers to DigitalOcean, and “factory” refers to their internal disk image building service. That’s just speculation though.