After signed up for your newsletter you mailing list should not disturb them by continuously sending newsletters. But you should send your first mail instantly after the registration. Then you have to keep a good gap between mails. It is said that an email that sends once in a month will get much attention.
The main objective of doing like this is to make a curiosity in the readers mind and they will wait for further mails. You should always use a mailing list to send emails to your customers so that you won't miss anyone. A newsletter is the best way mailing list to keep a relation with your customers. Always keep an eye on the length of the newsletters because if it's too long then it will be boring for them to read.
So keep it as short as possible. You should take all measures to improve the quality of your mailing list. Once customers find your newsletters mailing list useful then they will never think of any other service or products. You can use both email and regular mail to contact customers. But if you use regular mail it is going to cost you.