How?
setup SPF records in your DNS and then be sure to include language for opt-out/can-spam and then use
name<[email protected]>
for addresses. wont fix it all, but it can help
Also, make sure your server isnt ALREADY on a blacklist for spamming. And don't use shared hosting. If you try your best to not appear as a spammer on your account, somebody else on the server may not. The email providers will simply see the IP the mail is coming from and mark it as spam.
Many hosts will switch you to a different server if they do get blacklisted. Not to mention good hosts now work to stay off and get off those lists since they know it's a big deal.
Use Mailchimp, not your own mailing server. And don't send spammy e-mails.
Mailchimp is very costly. It's good if your forum is not too big. I too use SMTP relay currently from other provider.
Well his forum has 92 members, and Mailchimp is free up to 2000 subscribers ...
Although, if this is for activation e-mails etc., that's another issue.
Honestly, I would recommend using sendgrid.