Xenforo sends both HTML and plain text. If the email client doesn't support HTML, then it will automatically display the plain text version of the email.
certain servers such as yandex.com will disable links and images if the email goes into spam.
registered member can't click on email verification links. I need to send only plain text
The only way I'm aware of would do it manually. You could edit the email templates and replace the HTML content with the plain text version. Then you would also have to enable debug mode and do it for each email template.
Any templates edited manually would revert when updating unless you did it as email template mods in the form of an add-on.
@Sphregal you could do this to disable it being turned on by default for users: ACP -> Setup -> Options -> User registration -> Watch content on creation: Yes
ACP -> Setup -> Options -> User registration -> Watch content on interaction: Yes
That would automatically make users opt-out from email notification on signup. They have the option, through their account settings, to enable them if they wish though.
@Sphregal there is definitely an opportunity to write an add on which forces Xenforo to always send as plain text no matter what, since it already does that.
Out of curiosity, why are you interested in using only plain text over HTML + plain text in 2020?
@cmd I explained earlier
certain servers such as yandex.com will disable links and images if the email goes into spam.
registered member can't click on email verification links. so I need to send only plain text
There may be many reasons why an email may land in spam box.
Such a topic is too long to discuss here.
The main issue here is about a missing feature: the option to choose to send plaintext email or html email.
I am using a third third party smtp service because I don't want to go through hussle of email spams/blacklist etc
BUT that is not the issue here. The issue is I want to send plaintext emails