MySQL SELECT email where field contains a value

I would like to do a simple select - but the condition is a bit tricky for me (because I'm a SQL-beginner).

I got this table:

userid | email             | newsletters
     1 | test@example.com  | 1,2
     2 | test2@example.com | 1

Now I would like to get all email-addresses of users, which want to get newsletter "2".

This would be:

email | newsletters
test@example.com | 1,2

And of course: In another query all users, which are subscribing newsletter number 1:

Result:

email | newsletters
test@example.com | 1,2
test2@example.com | 1

What would be the correct sql-query? I think this should be the right beginning, but I don't know which condition I have to use:

SELECT email FROM users WHERE newsletter CONDITION?

Could you please help me out? :-)