That's a really good question to start with. But first of all, let's put things under the correct perspective :
Pressidium, offers a reliable and secure WordPress managed hosting environment that is not suitable nor it is intended for mass e-mail sending.
So, if you wish to use your WordPress site for activities like mass mailing or massive outbound campaigns, newsletters, etc then you should definitely use a 3rd party service like the one provided my MailChimp, GetResponse, Constant Contact and others.
For all other mailing needs, Pressidium has partnered with Mail Channels, a fantastic antispam mail relay service that will successfully deliver your e-mails at no time. So in other words, every WordPress site you host on Pressidium, will be sending emails automagically via Mail Channels (certain limits & restrictions may apply as MailChannels maintains strict policies).