The self hosted version of phpLists would be a perfect solution for your email campaigns.
https://www.phplist.org/download-phplist/The sends happen at the PHP level, rather than the HTML browser triggered level.
It is what I use on one of my UBB.threads forums. I've modified the stock code to replace the config option with a link to the user subscription page
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STOCK UBB.threads mail settings page.It looks like this:
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MODIFIED PAGEthe modification is not really anything complicated at all.
It looks like this:
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THE LINK goes to https:// EXAMPLE .com/lists/ This is the phpLists default user mail manager page: