BobDeBuilder wrote: have a local 2010 exchange server and tested sending to the smtp relay via powershell, works fine
The powershell was used on which server? Not the CentOS7 VM, right? If not, you might try it:
github.com/PowerShell/PowerShell
1. The mail account is a full exchange account (no shared mailbox etc..)?
2. You already checked if VM and Exchangeserver can reach each other via network?
telnet mail.example.com 25 from the CentOS CLI.
Depending on the CentOS setup you might block connections to the exchange server.
3. You tried an E-Mailclient (like Thunderbird) to connect to the exchange server?
4. Provide Screenshot and error log file (debug mode for email can be activated in LimeSurvey on the same screen where you enter the mail settings.