I opened a support case on this, but curious what others have found.
Our users are sync'd via LDAP. We also have tech accounts that are not LDAP'd, so Techs actually have two accounts with the same email address.
We've discovered that when their Tech account is inactivated, emails triggered to their regular active account fail (due to the other inactive account).
So apparently if an email address is associated with an Inactive account, any message to any other active account with the same email will fail.
According to Solarwinds (so far), this is how it's supposed to work. The present workaround is to put a dummy email address into the Tech account field, then we can successfully email their other active account. Doesn't seem logical to me..
On a similar note, I submitted a feature request to have Inactive accounts hidden from the autocomplete Recipient dropdowns so they cannot be selected when updating tickets, etc.