Probably due to a recent change in Sendgrid API, some test emails are not working (particularly from Automated). The problem is solved with our Sendgrid Addon 4.1.8 which has been changed to correctly send (valid) data to Sendgrid. Actually the data was already correct but not in the format expected by Sendgrid.
Please, update now.
If you don’t see the notice in your plugins panel, just save the Newsletter Main Settings to force a version check.