OK, I see this list of errors:
[2018-05-17 09:22:58] - Loading 90 campaigns, starting with offset 0...
[2018-05-17 09:22:58] - Found 22 campaigns and now starting processing them...
[2018-05-17 09:22:58] - Campaign Worker #0 looking into the campaign with ID: 3
[2018-05-17 09:22:58] - This campaign belongs to Joseph Then(uid: zb713p87o9374).
[2018-05-17 09:22:58] - Changing the campaign status into PROCESSING!
[2018-05-17 09:22:58] - Cannot find a valid server to send the campaign email, aborting until a delivery server is available!Campaign UID: qt321jodz83b3
[2018-05-17 09:22:58] - Campaign Worker #0 looking into the campaign with ID: 4
[2018-05-17 09:22:58] - This campaign belongs to Joseph Then(uid: zb713p87o9374).
[2018-05-17 09:22:58] - Changing the campaign status into PROCESSING!
[2018-05-17 09:22:58] - Cannot find a valid server to send the campaign email, aborting until a delivery server is available!Campaign UID: ls794t25yg9ab
[2018-05-17 09:22:58] - Campaign Worker #0 looking into the campaign with ID: 5
[2018-05-17 09:22:58] - This campaign belongs to Joseph Then(uid: zb713p87o9374).
[2018-05-17 09:22:58] - Changing the campaign status into PROCESSING!
[2018-05-17 09:22:58] - Cannot find a valid server to send the campaign email, aborting until a delivery server is available!Campaign UID: mn1051chbe184
Hmm.... cannot find a valid server to send.... What's the solution? The delivery server is the localhost itself.