HES rejected an email and you receive a Non-Delivery Receipt (NDR) with the reason "Too many hops".
The message "Too many hops" means that the email went through to several mail servers before it went to the last one that rejected it. This means either you set the hops threshold too low or the email was on loop.
For example, if a user has two accounts, forwarding to each other, it may results to an endless loop. A message sent to User1 account is forwarded to another mailbox; if that mailbox account is set to forward to User1, then a mail loop is created: mail goes from User1 to the mailbox, back to User1 and then back to the mailbox again, etc., until the loop is ascertained and the message returned to its sender.
To resolve the issue, check the NDR message received which usually contains information about the route or hops that the email has taken. It can be used to identify if the email met the hops threshold or it went on a loop. HES has a 26-hop threshold.