This value is used by qmail-inject to fill in the right portion (following the @ sign) of email addresses that don't already have something there. This applies to both from and to addresses. Since qmail-inject is invoked by the provided sendmail command, this also applies to any program that uses the sendmail command to submit emails. Many PHP (web) programs and some administrative functions (e.g. cron, logwatch) use the sendmail command and are configured with no domain in the sender's address, so you'll see this value appear in those cases.
man qmail-inject excerpt:
Default host name. Default: me, if that is supplied; otherwise the literal name defaulthost, which is probably not what you want. qmail-inject adds this name to any address without a host name. defaulthost need not be the current host's name. For example, you may prefer that outgoing mail show just your domain name.
The QMAILDEFAULTHOST environment variable overrides defaulthost. Texas liability insurance