Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
It seems like you are right, if the email under spam folder, all the link will become unclickable in Gmail, if I move to the inbox, that activation link will work just fine.
Unfortunately there is nothing we can do if it lands on the spam folder because it actually depends on your mail server. But I would suggest that using a transactional mail service e.g.
https://sendgrid.com/
It might be some of the possibilities why your e-mails lead into spam folder.
1. Your domain has been somehow blacklisted.
2. Your email server's IP address has been marked as spam.
Services like
https://sendgrid.com/ provides email delivery services and allows you to setup SMTP authentication in your Joomla global configuration. Most of the time, e-mails delivered by these transactional e-mails would lead into the inbox.
In the event that the emails are also flagged as spam, you could get in touch with their support to look into this.
For now you can try contact with your webmail provider regarding this and see if they encounter this kind of issue before? Because I noticed you are using SMTP mailer on your site.