If you think about it, how many merchants websites mailing list have you visited that have a trusted online and/or awards displayed on their landing page. They are building trust and authority. Your list building efforts must be niche targeted. It is nearly impossible to build a hyper-responsive list if your offer is general.
You cannot through in a general fishing net and expect to catch only monkfish. Your opt in offer must be niche and even sub niche targeted. Buying mailing list an opt in mail list is something I advise you to stay away from, tempting as it may seem. The reason why an opt in mailing list is so lucrative is because people have individually signed up to you and are therefore genuinely interested in.
Or curious about, what you have to say. If you buy an opt in list, the emails are not people who know who you are or have any interest in you. Yes mailing list there is the one off chance that one name in the 10000 emails you purchased may convert but this just isn't a cost or effort efficient method. You need to build your own list.