To avoid any delivery problems with mailings to your subscribers on the Mail.ru server, please, send only relevant and necessary letters to real people who are interested in them.
Double opt-in, authentication, unsubscribing from the mailing list, complaints about spam, non-existent recipient addresses and other issues.
These requirements are elaborated with standards developed and adopted by the participants of global working groups on combating spam, fraud and other forms of electronic abuse.
This article will be useful for email service providers (ESP), as well as for their customers. We also recommend you to check the general requirements for all senders.
If your service tends to import the address book of the user in order to send invitations later, you need to follow the rules and recommendations given here.