Spam from your own domain doesn't actually come from your own domain. We've seen this mentioned earlier here in the forums, and the brief explanation is that when DH mail servers receive a spam without a domain then the own domain is listed. This is not intentional; it's just the way mail servers work.
So don't worry that all that spam is coming from your own domain. It really isn't.
You should also turn on the Junk Mail filter for that domain. That will filter much of the spam away, though there are a few things to be aware of:
1) the aggressiveness of the filter can/must be set to your (mom's) preferences. Use the DH webmail to get into the Junk Mail configuration.
2) if set too aggressive, the filter may also catch legitimate mail. This will not be deleted right away. It's put in quarantine in the webmail's Junk Mail folder. You need to check that folder with regular intervals to make sure you're not missing mail, and possibly adjust the filter aggressiveness if you're not happy with the ham/spam ratio.
3) the Junk Mail can only be accessed through DH webmail, not other mail programs. If you're/your mom is using webmail anyway, then this is fine. Otherwise you need to make a habit of checking there, too.
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