Hi Claudio,
I can assure you that there is no general issue with something like that happening. It must be something specific to your site.
Let’s take one notification as an example and test it. Let’s use the “New Request Email” notification.
Go to the Notifications area of the Settings page and scroll down to the “New Request Email Subject” option. What does it currently say there? Have you input anything, or is it on the default? The default makes use of your site title. So, if you have some special characters in there, it’s possible that could mess something up. Try typing in something basic with no special characters. Then test again. Does that make any difference?
If not, then could you please let me know what you have set as your site title? We just make use of WordPress’s built-in wp_mail function to send the emails. By default, that may use either “WordPress” or perhaps your site title as the from name.
All that being said, you may need to look into using a plugin like WP Mail SMTP, which lets you route the emails that are sent via your real mail provider/server, and also lets you specify the from email address and name. This would have the added bonus of making your emails far less likely to be flagged as spam.
