Hi,
If it works for some and not others, it's indeed not a problem in HikaShop.
It could be that your emails are flagged as spam emails by some email servers.
You should check that your email server is not blacklisted, and if possible, configure SPF and sender id.
More information on all that can be found here:
www.acyba.com/en/support/documentation/108-acymailing-spam.html
But you should contact your hosting provider so that they can help you understand why some of your emails don't arrive to their destination.
You could also simply deactivate the user account activation email in the options of the user manager to circumvent the problem, at least temporarily.