I can’t speak for Caldera forms with any certainty, but I can confirm all is well with Divi forms.
Don’t forget, Post SMTP is basically transparent to any other plugins and themes (other than those that contain their own mailing functions).
The idea of Post SMTP is to replace the in-built PHP Mailer functions that a basic WordPress installation would use. PHP Mailer uses your servers PHP to act like a mail server, which can cause issues as these are often abused, so mail sent this way can be treated as spam by recipients.
Using Post SMTP allows you to override the WordPress PHP Mailer function and replace with a number of different ways of sending emails. (i.e. SMTP server where you specify the mail server, username,password, or Gmail/SendGrid/Mailgun etc etc).
Those are much more reliable ways to send emails (providing you have them set up correctly that is – using SPF/DKIM records).
The use of Post SMTP is 100% transparent to all other plugins. The other plugins just create an email and send it to WordPress for sending, if Post SMTP is installed, that sends the email instead of the inbuilt WordPress method. As I mentioned above, the one caveat is IF the other plugins have their own mailing functions built in (i.e. it has its own SMTP method to override PHP Mailer).
