Activation email not sent to specific email address

Hi,
Failed to add two specific email addresses. An activation email was just not sent to them.
According to a check in the firewall, the email is not blocked and does not enter spam. IPs 198.2.179.123, 198.2.179.168 and 198.2.179.86 already in whitelist in FW. I also do not see any attempt in logs to open incoming session from these addresses. 
All other users in the same domain have successfully activated their accounts.
I also have two extra licenses so it is not a problem of licenses.