First,  make sure your email server's spf record includes the Achieve IP address. See This prevents your school's email from being tagged as "spoofing".

Ask the user to check their junk/spam folders.

Ask the user to add your school's email domain to their safe sender's list. Depending on their email client, the user may be able to add a filter to "never send to spam" or "never block sender" or "never block sender's domain".

IMAP and POP are both ways to read email from an email service (such as Gmail) in other email clients (such as Outlook.) In other words, it is common for a user to have multiple email accounts sending to Outlook, Gmail, etc. Even though the user may add your school's domain as a "safe sender" on their email client, the  "safe sender" or filter must be added to the originating email service.

If users are unable to receive email containing their temporary portal password (in order to login to the portal), try going to Website=>Users and look up the parent. Click the icon to the left of their name. Copy the temporary credentials. Create an email using an email account outside of Achieve and paste the temporary credentials into your email message - and send.