MIB seems to take a very long time to deliver email to outside destination

Using PHPMailer() in my web app, I send an email alert to a user through one of my MIAB accounts. It sometimes takes an hour or more before the user receives it in her Office365 hosted account.

I can see in my PHP logs that the email is sent (as far as my app is concerned) immediately.

I wonder if this is because of server time differences? The web app runs on UTC and MIAB is UTC+5.

Any other suggestions of where to look? MIAB is hosted on a DO Droplet if that matters.

Is PHPMailer setup to use MIAB postfix?

Did you ever resolve this? I’ve had similar issues (unresolved, hence coming to MIAB) with other web servers (on shared hosting). I think Office365 is rather aggressive about who it receives email from.