This is a frustrating Magento 2 issue whereby the "Sender" information in an email is actually removed/stripped from the email that the customer receives.
This is bad as most email clients such as GMail will simply reject this email, meaning your customer will never get notified of their order!
The reason for this is that Magento 2.2.5 (and possible 2.2.4) comes pre-packaged with a few 3rd-party modules which interfere with your SMTP extension.
In order to rectify this and get your email header back to normal you should disable these 3rd-party modules.
php bin/magento module:disable Klarna_Core Amazon_Core Amazon_Login Amazon_Payment Klarna_Ordermanagement Klarna_Kp Vertex_Tax
Of course, if you're using some of these then you'll have to pick and choose which ones to remove. I'm not 100% sure but I think the culprit is the Amazon_Core extension, so if you're using Amazon Pay you'll probably have to implement a different fix.