Our notification messages, with relatively complex verification links and license activation codes, may look 'strange' to email services designed obsessively, such as Microsoft email services. They may block these messages and not others which look more 'normal' to them. On the other hand, we use SparkPost to send masssive email messages. SparkPost is an important company which is much more difficult to block with impunity. We are very small and European. Draw your conclusions...