Unable to send e-mails to Seznam.cz, Host not reachable

Unable to send e-mails to Seznam.cz, Host not reachable

Hi Zoho Team,

since yesterday I'm unable to send e-mails to seznam.cz or email.cz domain (same provider). It returns a mailer deamon email with error: ResponseCode 421, Host not reachable.

Is this a temporary issue or are there any long-term problems with Seznam.cz?

Thank you for your help,
Monique