If you can’t send an email message: you receive an error message “Delivery Failed” or,
– Emails remain in the Outbox.
1. Internet connection is down: check your connection.
2. Issue with the recipient mail service provider: If you only fail to send mail to one specific email address, it may be an issue with their email service provider, that has an issue receiving email, not that yours is failing to send it. You can check this by sending some quick test emails to other email address and check the receiving status.
3. Check your firewall or antivirus settings: you can temporarily disable any firewall, antivirus, or antimalware application, then check your email again
4. Attachment size is too large to send: If the email failing to send has a large file attachment, try sending a test email with no attachments. Generally, a file attachment should not exceed 35MB.