@Anatoliy I'm not a big expert either. But what I understand is that spammers can create as many domain names as they want and flood mailboxes using SMTP addresses from their domain names, so sending emails via your own server presents the risks being considered a spammer. Third-party services are identified as trusted providers because they will detect any suspicious email sending internally. If you subscribe to one of these services and start spamming then you will have a good chance of having your account blocked/deleted.