If you have a website, which collects visitors' e-mail addresses, then you should maintain contact with those registered on the site. This can be in the form of weekly newsletters, promotional offers and other related campaigns.

If you make use of WordPress platform, you should install plugins such as WordPress Newsletter Plugin to send mailers. I have seen several plugins which have the ability to send mass mails all at once. However, this process will drastically affect the functioning of the mail server. Hence, you require an automated solution, which has the ability to send a fixed number of e-mails on an hourly basis without causing any heavy loads on the server.

WordPress Newsletter plugin developed by Tribulant Software ships with several advanced features such as Multiple Mailing Lists, Bounce Management, Gmail SMTP Capability with several themes. Moreover, you can directly import subscribers using the Import/Export feature and automatically pump new subscribers to the mailing list. I feel that this feature will be very useful for blogs and WordPress article directory websites that are already established on the web.