This is similar to 0025606
It's not caused by Mantis, see 0025606:0061709.
Maybe it's caued by a 3rd party plugin, your email client, or your email provider

but 0025606 was not resolved - there are no any solution, exept "we don't know what is it".

Maybe it's caued by a 3rd party plugin
As it was mentioned in 0025606 (and in my case too) this problem appears with clean mantis installation and continues with plugins or without them.

your email client
This issue appears in several different clients and with several users (actually with all of my users)

or your email provider
My provider is myself. I used exim+dovecot with standard configs and there were no problems with other software anytime, excpet this.

Maybe this problem needs deep research?
From my side i could provide any help and information if needed...

also linked with 0010088 0010369 0010401

Did you try what's been suggested at 0025606:0061719 ?

I need a time to check, please don't close this issue...

I can't uderstand

https://github.com/mantisbt/mantisbt/blob/release-2.20.0/core/email_api.php#L537

Line 573 as sugested in https://www.mantisbt.org/bugs/view.php?id=25606#c61719 or line 537 as we can see on link?

Anyway my mail table is empty after i try to create a new user and emails still sent (and still in HTML instead of plain text)...

Line 537 is the right one.

Maybe you did not understand what was meant with remove the force-sending of signup e-mail
It means change
email_store( $t_email, $t_subject, $t_message, null, true );
to
email_store( $t_email, $t_subject, $t_message, null );

Пользователь XXXXX создал аккаунт для вас с именем пользователя "XXXXX". Для завершения регистрации, перейдите по следующей ссылке (проверьте, чтобы ссылка была в одну строку, иначе она не сработает) и введите в соответствующие поля удобный для вас пароль доступа (будьте внимательны, пароль надо ввести в два поля):

<a target="_blank" href="https://XXXXX/verify.php?id=10&confirm_hash=XXXXX">https://XXXXX/verify.php?id=10&confirm_hash=XXXXX</a>

Если вы не запрашивали никакой регистрации, проигнорируйте это сообщение, и ничего не произойдет.

НЕ ОТВЕЧАЙТЕ НА ЭТО СООБЩЕНИЕ 

There are HTML tags in emails table

Are you complete sure that you don't have installed any 3rd party plugins?

Please provide screen shots (with English language settings) of the plugins page and the settings of the MantisBT Formatting plugin.

hm... i leave only official plugins but in plugins list i found bbcode plugin (don't know how can i miss it).
when i disable it - everything works as should.

Thank you for your pation and, i think, this information should be pinned somwhere...
Also i try to contact with developer of this plugin and inform about this issue.

Follow-up in BBCode+ plugin https://github.com/mantisbt-plugins/BBCodePlus/issues/89