'Could not instantiate mail function' error

Dear Sir or Madam,

Hi, I’ve had a problem with sending a newsletter.
I duplicated and amended my previous one, as I usually do, but when trying to send a preview to myself I get the message:

“Error Sending Message […my newsletter title here…] to jembowdenwatercolour@gmail.com | Could not instantiate mail function.”

I’ve also tried bypassing the preview email and just sending the newsletter to my mailing list, but it doesn’t work – gives the same message.

I haven’t changed anything about my email set up, and as I say, the newsletter itself was just a duplicate of my previous one which sent no problem (as far as I am aware!).

Please can you help?
Many thanks and I’ll look forward to hearing from you.

Yours faithfully,
Jem.

https://wordpress.org/plugins/wysija-newsletters/