This is an odd one. Emails sent from the "Mail members" feature added by the `group_tools` plugin do not arrive at the intended members. Here's what I've observed (so far):

• the number of members targetted for the email is correctly displayed;
• having clicked the "Send" button, a success message is echoed: "Message succesfully send" (should be "Message succesfully sent", of course - but that's not an issue!);
• in the Administration Dashboard, the "Content statistics" displays the correct number of "Group mail" uses;
• BUT ... no users receive the email.
• However, emailed notifications are working correctly (so it seems the server has no problems generating and sending mails).
• I get the identical results on two different installations of Elgg on two different servers.

On our production site, we are using Elgg 2.3.8 on a (virtual) server running PHP 7.0. I've looked in the "Group tools" issue list, and can't see anyone else having this problem.

Help diagnosing this problem (or even better, fixing it!) would be appreciated. I've looked at all the settings I can think of to look at, but can't spot anything amiss -- but I'm well aware that I've got a lot still to learn about Elgg and its ways! Thanks.