My outgoing server settings are correct. Why doesn't it work

My outgoing email server settings are correct. The test gives an error or ocassionally tests fine! I have the test emails to proove it!

I use the same settings on Postman SMTP plugin and it works fine.

Yes I have deactivated Postman SMTP to test to see if there was a conflict - No.

Can someone please help me I have been trying to get out my enewsletter particular welcome to new subscribers for days now!!

Cheers,
Kelly