New SSL certificate was installed on the server to secure mail.
iPad and iPhone users are unable to send or receive emails.
iOS operating system intentionally blocks some of SSL certificates for security reasons.
There are two possible solutions:
- Recreate email accounts of on iOS devices.
- Allow using required SSL certificates from device settings:
- Go to Settings > General > About > Certificate Trust Settings;
- Find a section called "ENABLE FULL TRUST FOR ROOT CERTIFICATES";
- Choose Plesk server mail certificate from the list and enable it.
Note: Interface on different versions of iOS may vary.