Newsletter signups with "." in the address don't work

so I noticed that Newsletter sign ups don’t work, if the user has a . in their address.

Error: Please enter a valid email address!

I looked at some posts and I couldn’t really find a solution to the problem. How can it be fixed? Is it some backend issue or a theme related issue?

Blog URL:
Ghost Version: 3
Theme: Lyra
Hosting: DigitalOcean Droplet

Check server logs. This may indicate that your mailing service is unable to send transactional email (due to misconfiguration or whatever). For some reason Ghost show “incorrect email” error in such cases.

You can test such emails on my test page: here I was able to subscribe with emails with dots.