OpenCart email not sending

My OpenCart site uses SendGrid as my email sending system. All of this worked until recently. When using features that require email to be sent to the admin (e.g. new order notification emails, creating a new account), I get this error:

 Notice: Error: Password not accepted from server! in .../system/library/mail.php on line 251

      

        
        
        
      

    

In OpenCart Settings > Mail

my settings are as follows:

Mail Protocol: SMTP
SMTP Host: smtp.sendgrid.net
SMTP Username: <username>
SMTP Password: <pass>
SMTP Port: 25
SMTP Timeout: 5

      

        
        
        
      

    

This is according to SendGrid documentation: https://sendgrid.com/docs/Classroom/Basics/Email_Infrastructure/smtp_ports.html

Can anyone advise? These settings are working on my localhost. I contacted the host to check if they were blocking the SMTP port, but they replied that they have no way to block the port as the SMTP host is not their website, but SendGrid instead.