How to legally make a one-time mailing list?

Question

SMTP

There is a base of 500 mail addresses, I want to send out letters so as not to do it manually.
I created 500 variants of letters and wrote a script, now it remains to understand how to do it legally without harming yourself.
As I understand it: if you do mailing via SMTP with several mail accounts (on mail services like mail.ru), using a timeout and different letters (diluted with synonyms), it will look legal, as if I send them manually to each separately or all this is not an option, since my IP address will be taken down and they can complain?
PS: at the end of the letter there will be a URL address
. UPD: I used the services, but they banned sending "Reason for blocking: Distribution according to lists collected on the Internet is prohibited."