Opensmtpd not starting on bootup

[Jane Atkinson]

I have opensmtpd installed on my desktop PC so as to get emails from cron etc.

Since I upgraded to Xubuntu 18.04, opensmtpd won't start on bootup.

systemctl status opensmtpd shows:

● opensmtpd.service - OpenSMTPD SMTP server
   Loaded: loaded (/lib/systemd/system/opensmtpd.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Wed 2018-05-23 12:25:22 NZST; 10min ago
     Docs: man:smtpd(8)
  Process: 807 ExecStart=/usr/sbin/smtpd (code=exited, status=1/FAILURE)

May 23 12:25:23 ja-desktop smtpd[807]: invalid virtual ip or interface: enp3s0
May 23 12:25:22 ja-desktop systemd[1]: Starting OpenSMTPD SMTP server...
May 23 12:25:22 ja-desktop systemd[1]: opensmtpd.service: Control process exited, code=exited status=1
May 23 12:25:22 ja-desktop systemd[1]: opensmtpd.service: Failed with result 'exit-code'.
May 23 12:25:22 ja-desktop systemd[1]: Failed to start OpenSMTPD SMTP server.

enp3s0 is definitely a valid interface; the only thing I can think of is that it hasn't been defined at the time opensmtpd tries to start.

I can start opensmtpd manually once I've logged in.