Sending test emails SOMETIMES fails

I recently performed a parallel update from phplist 2.10.12 to 3.0.12. We kept the old version operational while the client became accustomed to the new one in demo mode. Today we switched to 3.0.12 and turned off demo mode, and we’re having difficulty with the test emails.

phplist is configured to send two test emails, one in text and one in html. When sending a test to me, both emails are sent (and are put in spam), and phplist announces that they were sent successfully. When sending a test email to the client, however, only the text message is sent successfully. When changing the setting to only send one email, phplist announces that sending failed, and indeed no email shows up.

The client is indeed capable of receiving html emails at that address, and the error log says “Error sending email to [email address] Could not instantiate mail function.” However, to reiterate, it did send correctly to me - twice. But it fails every time sending to the client. PHPMAILERHOST has been left blank, so it should be using mail(). I do not have easy access to the email or server error logs.

I am unfamiliar with configuring phplist, and any help here would be appreciated. Thanks!