PHP List says it sends mail, but it actually doesn't

Hi there,
I get this message after I sent out a mailing:

11,273 Messages sent on 23 June 2021.
0 Viewed (0,00%), and 0 bounced (0,00%).

I didn’t get the mailing myself, nor did I get the test mail. I looked all over, but I don’t know what I’m doing wrong. It used to work just fine when I send my previous mailings.
I hope someone can help me locate the problem.
Much appreciated!