So from the Apache logs I got this:
[Fri Oct 19 15:01:35.351367 2018] [:error] [pid 10132:tid 1524] [client ::1:52309] PHP Warning: dns_get_record(): DNS Query failed in C:\OsTicket\OsTicket_Server\www\include\class.validator.php on line 164, referer: http://localhost/scp/users.php?do=import-users
Are email addresses verified against DNS records?
Looking at include\class.validator.php suggests they are.
In that case, that might explain my error above as some Googling did indeed reveal the address domain to be incorrect...