What is the difference between Swift and PHP mailer?

Question

Email

With the same setting of the sending limit, a noticeable difference is visible.
But what is the fundamental difference technically?
Sending Program script ---> external SMTP ---> recipient
Sending speed (intuitively we understand) the script seems to cheat almost the same, and fly the same during tests, the general dynamics suggests:
- Swift: Slow
- PHP: Fast
Auto-replies for sent letters
- Swift: No
- PHP: A lot
of Spamming and spam tags (I watch mail)
- Swift: No
- PHP: Or are there Auto-replies or appear during the day
?! Out of 1000 php - 40 will be with Swift - 1 / one setting, change only sw/php