Activation email being sent as spam?
Are you using PHP or SMTP?
Go to ACP>Configuration>Settings>Mail Settings
If it's PHP then it could be that your hosts IP is blacklisted or reported for sending a lot of unsolicited/SPAM e-mails. If you are using SMTP then you have to contact the mail provider. In general it's not a MyBB issue. It's your host issue if your mail handler is PHP
Go to ACP>Configuration>Settings>Mail Settings
If it's PHP then it could be that your hosts IP is blacklisted or reported for sending a lot of unsolicited/SPAM e-mails. If you are using SMTP then you have to contact the mail provider. In general it's not a MyBB issue. It's your host issue if your mail handler is PHP