Your website can send emails to users about their accounts - for example, if you've opted to send a notification email when an account is created, or if the user has asked for a password reset. Sometimes, an email from the website doesn't show up in the user's inbox - it may be marked as spam, or the server might be restricted from sending these emails.
Check spam folder
The email may have ended up in the user's spam folder.
Ask them to check there, and add the email address it was sent from to their contacts. This will ensure future emails (such as ones for password resets) won't end up as spam too.
Change the SPF record
If the user still hasn't received an email, the issue may be that the web server isn't allowed to send emails from the specified email address. You can see which email address this is by logging in to your website and going to the Configuration section - the email address will be in Site information.
To allow the server to send emails from your email address, you will need to authorise it by setting up a Sender Policy Framework (SPF) record with the server's IP address.
An SPF record is a TXT record that lists the mail servers that are allowed to send email from your domain. This is used to prevent spammers from sending unauthorised emails from your domain - emails sent from a server that isn't on the SPF record might be marked as spam.
If you use Google's G Suite, you can see instructions for how to set up your SPF record at https://support.google.com/a/answer/33786?hl=en
For Office 365, see https://docs.microsoft.com/en-us/office365/securitycompliance/set-up-spf-in-office-365-to-help-prevent-spoofing
If you don't use Google or Office 365, you can Google instructions for your specific DNS registrar.