Which email addresses do you want to make conditional?
Say for example you want to have emails sent from sales@yourdomain.com , help@yourdomain.com , enquiries@yourdomain.com etc etc, then the way to do this is to set up your Post SMTP to use something like outbox@yourdomain.com
You won’t hit any issues with emails being marked as spam, as all emails will originate from @yourdomain.com
However, if you want to send emails from user@domain1.com, user@domain2.com, user@domain3.com (i.e. sending via different domains), then it gets a little trickier because you’ll be sending those emails using the account set up in POST SMTP (i.e. something like outbox@yourdomain.com)
In this instance, providing you have access to the domain zone files, you can authorise yourdomain.com to be able to send email “on behalf of” domain1.com, domain2.com, domain3.com which again will reduce the likelihood of emails being trapped as spam or rejected.
So it all comes down to which emails are conditional, and where they are used.
Post SMTP should be transparent to how you set your site up.
Even if you were not using Post SMTP and you used either of the above scenarios (same domain, different email accounts or multiple domains), the settings you would make to improve email deliverability would be done at server level, not at WordPress level.
Can you clarify which multiple email addresses you would like to use and where they are used? (i.e. different sending accounts, different sending domains, different recipients, or a combination thereof !)
Regards
Chris
