I am using Drupal 7.56, CiviCRM 4.7.20 and Mosaico extension 2.0-beta2.1494950412. When sending an email with a few images to ~950 contacts I get an Internal Server Error (500). Removing a few images and the email succeeds. The server is presumably running out of some resource but I have not been able to determine what it is. Is anyone else experiencing this issue? If so do you know what resource I need to increase (if indeed that's the problem)?
The problem is caused by insufficient php memory. My php memory setting was 256MB and when I increased that to 512MB the problem was resolved. I don't know what setting would be 'correct' since I am not keen to retain php memory at 512MB.
Whilst increasing php memory solved the problem I was not comfortable in leaving the memory allocation just for Mosaico. So I throttled down the number of emails being sent at a time and reduced the php memory back and I have had no issues since that time. My settings are now php memory 256MB and I send 250 emails at a time.