SMTP is a mail delivery protocol only and his usage can ensure the correct delivery of your Sendit newsletters. There are some basic configuration to know in order to send mail via SMTP. First of all you have to know hostname, port, authentication type (ssl/plain/tls). Each hosting provider has normally his own settings, so before to setup ask to your prvider for smtp relais, params and limit. You can also test your email from localhost enviroment locally also connecting to external smtp services. I used and worked with a lot of popular hosting provider from italy and u.s.a. and i got some tipical configurations to share with you. Basically the best way is to use port 465 and SSL connections, should work on 99% also with the MAMP / Localhost enviroment! If you are not familiar it’s hard to understand exactly how a SMTP Relay Service Provider helps deliver emails, but basicallly it’s like an identifier for you a special signature for your emails with neutral IP and a great “reputation” (to avoid the spam risk).
There are a lot of SMTP external relay service, and i will put some configuration example to use with Gmail Smtp / Smtp.com / Sendgrid. Basically Sendit allows you to decide and set your SMTP provider overriding all email wordpress settings via phpmailer, so if you setup smtp all your email will use your Sendit Smtp Settings.
I am in partnership with SMTP.con, one of most popular and effordable smtp service of the world.
Get an account on Sendgrid following this link (also free account) if you want just to test, and setup following settings: