Thanks guys!
Hi to both of you, many thanks for your help - I am embarrassed to say that I hadn't actually clicked on the e-mail 'sales@mysite.com' in the Admin Panel, 'Email' tab... I do apologise. I was looking at the 'Settings' page before, and I thought that I was supposed to set the e-mail up from within there, under the 'Email settings' section (which sort of made sense, before I found the other email section!)
Many thanks guys, I've got it all working now, beautiful.
Just in case anybody finds this thread and needs help, I did the following (substitute your website for "mysite.com" in the instructions below)
1) Click on the Emails tab, in the Admin Panel. (You should see the tabs
'Dashboard Settings Emails Help Topics Staff Departments' when in the Admin Panel.)
2) Click on the email address that you want to use to send email replied to customers. In my case it says
"Support"
3) Under "Login info (optional)" enter
Username: sales@mysite.com
Password: enter your email password
4) Under "SMTP settings (optional)" enter
SMTP host: smtp.fasthosts.co.uk (you will need to contact your hosting service's help desk if smtp.yourhost.co.uk or whatever, doesn't work)
SMTP port: 25
5) Click 'submit'.
Make a test ticket using another e-mail address of yours (your personal e-mail, for example), then post a reply, and you should receive the reply in your personal e-mail address inbox!
THANKS GUYS!
I love OSTicket, a great solution, only took me twenty minutes to set it up, and that was the first support ticket system I had ever installed on a server, so I'm very pleased with it.