The issue has been found and fixed, our webmail is operable again via mail.vnav.cloud.

Due to a technical issue, our webmail is currently unavailable. However, VNAV-hosted email addresses can still be used through an external mail client.

Connection Details
Username: your email address (e.g. example@vnav.nl)
Password: your normal webmail password

POP Service
Address: mail.vnav.cloud
Port: 110 / 995

IMAP Service
Address: mail.vnav.cloud
Port: 143 / 993

SMTP Service
Address: mail.vnav.cloud
Port: 25 / 465 / 587
We are currently working on a fix and appreciate your patience.