Normally I would suggest that the machines were holding some old data and that it would eventually get sorted out when all the DNS servers updated etc etc. But this is beyond that.
To be sure I am getting this. Let us say the person was named Jessica Garcia (Jessica.garcia@domain.com) , got married and became Jessica Johnson (Jessica.johnson@domain.com)
I am also assuming that in Outlook, when in the GAL, Jessica Garcia is still there (for those who have no idea she was married) so that both names can be found, and that if you search for Jessica Garcia, the query will result only with Jessica Johnson, so the email will send.
So what happens at the machine(s) if this scenario is correct and you send an email to Jessica.garcia@domain.com manually (not the new address) vs sending to Jessica.johnson@domain.com
How about the webui of the devices , what shows in a search for the user?
On the 78xx, does anything change if you go to the device, login, select Machine Status > Tools > Troubleshooting > Resets >Software reser > Reset Both ?
Since you have access to the LDAP server, can I assume you also have access to the SMTP server? If so, what is the server side showing when the machines are attempting to send to her?