I assume you mean that any mass mail message that you send isn't automatically flagged as spam by the receiver's mail client (or their mail server or their ISP).
The best advice? Use a mass mail service with a good reputation and "swallow" the costs. Tariffs are usually based on the amount of recipients you want to address. But by all means, use a service with a good mail reputation, else your message will end up in the recipients spam box anyway.
If you want to use your own mail-server for this, better think again. There are a lot of organizations that keep a very watchful eye towards mail traffic. And if they see a mail-server they don't know about, pushing a lot of mail messages out in a whatever period they deem short, your mail server gets flagged and you must request them to lift this flag. Until they do so, don't expect any mail from your server to arrive anywhere.
SpamHaus is a good example of such an organization, but there are many, many more of these organizations. It can take a week or two before you are off their shit list. However, if your server is again flagged as spammer in a grace period they define, getting of that list will take a lot longer. Do this often enough and your mail-server gets a permanent ban.
Now I say the whole time: 'your mail-server' but you can substitute that term with 'your domain' but that won't change the story. Getting flagged as spammer is easy, getting rid of that is time consuming.
If you are still hell bent on using your own mail server, make sure it is configured correctly. And that takes a whole lot more than just "install this (free) mailserver software and you're done." There are so many reasons for your message to be undeliverable. And some mail servers configured to be very picky about what they deliver to their users. It usually takes quite some time, money and expertise to make mass mailing an option.
Actually, more often than not, it is cheaper to send mass mail messages through a mass mail service than to do it yourself. Besides this, mass mail services come with tools that let you create your mass mail message very easily and you can be certain that the message will show up nicely on whatever device the recipient receives your mass mail message.
Making a nice looking HTML formatted message yourself that shows up correctly in webmail (browser), mail clients on PC/Mac/Linux, mail clients on Android/Apple/MS tablets or smart phones...that is a serious exercise in frustration. Not to mention time-consuming.
Honestly, if you don't do weekly mass mailings to a big amount of recipients, the headache and frustration of doing this yourself isn't worth the measly sum you have to pay to mass mail service to do this for you.