Problem sending confirmation emails

[mustards]

We have VM3.0.14 running on Joomla 2.5.28. It was working fine, sending confirmation emails out until recently (a few days ago) - no changes to any code. The site is configured to use PHP mail.

To test the situation I created a dummy order and I change the status to force a resend of the confirmation email. When I try this I get "could not instantiate mail function". However if I send a test mail from Acymail (which is configured to use the PHP mail setup) it works. I can also create a contact form which results in a mail being received, again using the PHP mail setup. I can also use a test PHP program to send an email, confirming that the server settings are ok.

Other than the usual reasons for this not working (e.g. PHP mail not working or not setup correctly) can you advise on what I can investigate? Alternatively, is there any way to use Acymail (or any other package) to send the confirmation emails, since this works?

Thanks

[jenkinhill]

If you made no changes, then we would assume your host made some changes to the server. Check the server error logs for an error report, and turn on VMdebug in Configuration/Shop/Advanced Settings to see if that shows the issue.

[mustards]

Thanks, but if it were a server issue then presumably the contact form and Acymail wouldn't work either, as they both use the same infrastructure?

[jenkinhill]

So what does debug say?

I don't use J2.5.x but there are a couple of threads of interest: 
http://forum.virtuemart.net/index.php?topic=112193
http://forum.virtuemart.net/index.php?topic=124646.msg442814#msg442814

[mustards]

No debug messages from Joomla itself. I turned on the VM debug and got the attached, which didn't have any specific messages.

Is there any kind of VM-related (or Joomla-related) debugging I could do to see what the problem is?