Hi kejabe,
I have the same problem with Microsoft and AOL blocking my website mail-server IP.
I installed this plugin “Postman SMTP Mailer/Email Log” and it’s blocking me from sending emails. I’m hosting with siteground and use their emails server. I heard that many people use Google APP email.
How did you fix your problem using this plugin? Is there any special configuration that I need to do? when I installed the plugin I just used ‘Start the Wizard’ and follow the instructions. Thank you.