How can I set a default email domain name on debian?

I have exim4 configured on my server as an email forwarder (for example sending an email to [email protected] will result in the email being forwarded to my personal gmail email account).

This setup works perfectly fine, although things start to get annoying when I went to send an email directly from the server using mail. For some reason, mailx always uses @debian as its domain name. I can fix this by manually specifying the sender when using mailx (I usually just do something like echo "yo" | mailx -s "hi there" [email protected]), but that seems unnecessary as I'm able to send emails properly from a friend's server without having to specify the ender email address.

One thing I did notice is that I had forgotten to configure the /etc/hosts file when I initially created the server (had torn down a vps a rebuilt it). So I did that and reconfigured exim just be safe, but that did not have any effect.

Here is what I currently have in my update-exim4 file:

dc_eximconfig_configtype='internet'
dc_other_hostnames='NoneOfYourBusiness.org;NotForYourEyes.me'
dc_local_interfaces=''
dc_readhost=''
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost=''
CFILEMODE='644'
dc_use_split_config='false'
dc_hide_mailname='false'
dc_mailname_in_oh='true'
dc_localdelivery='mail_spool'

Would any have suggestions for correcting this behavior? Thank you