@Tutanota I've had similar issues when using ProtonMail as well. ProtonMail gets around this with aliases, will you be using a similar strategy or something else. To often when a service is security minded a conservative IT manager incorrectly assumes its hackers and criminals that use it.
@Sh4d0w_H34rt Yes, you can use aliases with other domains if you are using Premium. We'll contact them, we're sure we'll find a solution :)