When Zoho Mail retrieves the emails from your POP server, it retrieves the email using the password you have provided when configuring your account. In case the password has changed in your server or the password has expired, the server returns an authentication failure error.
If there are subsequent failures for 10 times, the account will be automatically disabled. You can edit the account and the account will be re-enabled once you provide the correct password in the Mail Accounts page. Refer here for instructions to update your password.
Connection failure when retrieving emails
When Zoho Mail retrieves the emails from your POP server, it first connects to the server in the custom port number you provided when configuring your account. If your email POP server is down or not available or refuses connections, the connection failure occurs.
If there are subsequent connection failures for 50 times, the account will be automatically disabled. You can edit the account and re-enable it after you make sure that the server is available in the given port number. Sometimes, it is possible that your POP server refuses connection to Zoho Mail servers alone, in which case, you will be able to access the POP via other clients like Outlook. Hence you need to communicate to your Email Administrators that you are accessing your account via POP in Zoho Mail and hence allow Zoho Mail servers to access your accounts.
Certificate error - PKIX Error when retrieving/ sending emails
If your server support SSL, but provides an invalid certificate, you may get an 'Invalid Certificate Error'. You may need to contact your email provider, to check if the server has a valid registered SSL certificate from a certified Vendor.
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If your server has a self-signed certificate, you need to get a standard and valid SSL certificate from a vendor, without which you will not be able to configure your account in port 995.
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If your provider uses a standard SSL certificate, and you still receive Certificate Errors during configuration, please contact support@zohomail.com with your Mail Server Details and port number.
SMTP Authentication failure
Authentication failure when sending emails through custom SMTP
When you send an email from your external email address through Custom SMTP Servers, the email is sent using the chosen SMTP Servers. Certain SMTP Servers require authentication while sending emails. If the password set up in the server does not match with the password provided in Zoho, you may get an authentication failure error when sending emails.
In this case, you need to update the correct password in the ‘Send Mail As’ section in the Settings page.
Authentication failure for Office 365 accounts
If the SMTP authentication fails for the Office 365 account, you need to check if SMTP Authentication is turned ON, in addition to password and Two-Factor-Authentication (TFA). You need to contact your Office 365 Admin to enable the setting.
You can refer this document to enable SMTP Authentication in Office 365.
If your Office 365 account is linked with GoDaddy, refer to this document to enable SMTP Authentication.