Notifications not going to all users

I’m using PublishPress for a site with low traffic. I’m sending synchronous notifications. I have about 75 target recipients for post notifications.PublishPress is never able to send to all recipients.

Notifications always fail for between 8 and 12 users. The same users are frequently (but not always) affected. Email domains for these users are not consistent (yahoo, hotmail, gmail, and others are always involved). The users are in the same groups as users who do (always) get the notifications. I can send manually to these users through a regular email client.

I can’t see a pattern, and I’ve found no proper error message.

HELP!!