Yep, also using SparkPost, but they say I'm not blacklisted ...
Running on smtp settings:
mail_driver: mail
mail_encryption: STARTTLS
mail_from: noreply@censored
mail_host: smtp.sparkpostmail.com
mail_password: censored
mail_port: 587
mail_username: SMTP_Injection
BUT, I also tried removing all those things and left only mail_driver: mail.
BUT, I also have the same settings in postfix (for connection with SparkPost)
So, yes, related, but no, not fixed with those suggestions