It happens when sending to various e-mail addresses and it seems to be random - the message is like this:
This is an automatically generated Delivery Status Notification.Unable to deliver message to the following recipients, due to being unable to connect successfully to the destination mail server.
The message that you’re sending out is getting marked as spam due to issues with the account or the content of the message. To resolve this, check your account settings as well as the inbound spam filtering on the recipient’s side.
Mass Mailing: The most common issue is that a message is CC’d or BCC’d to a large number of recipients, which is similar to how spam messages are sent out. Therefore, our system may confuse the bulk mail sending with spam. We recommend that you use another option such as Google Groups to send out messages to large numbers of people.
Please note that a group is placed into bounce state after messages can't be delivered to a large number of the group's members. Check that your Google group is sending messages to active email addresses and that the addresses are spelled correctly.
Third-party blacklists can cause your messages to be rejected by other domains. You'll need to contact their domain administrator and ask them to configure their mail settings to accept your mail. Their settings could be configured to reject messages that aren’t sent from specific IP addresses or servers, which will prevent your message from going through.
Account issues: If you’re using a POP/IMAP client (like Outlook, Thunderbird etc.) , sign out and then sign back in to your account using the web interface (from Internet Explorer, Safari etc.) . You may need to reset the CAPTCHA for security purposes, to make sure your account is not a victim of automated spamming. You can reset CAPTCHA by following the instructions here. If your account has been mistakenly disabled for spamming, contact support to reset your account.
Inbound Gateway and server configuration: In your control panel, if you have "Only let my users receive mail from the email gateways listed above" option selected, and you don't have the IP address of a legacy server listed, then you will have problems sending email to users at your domain.
From my experience, I can tell that, the problem is on the recipient side and not on your side, please let me know, if you need any further help, otherwise please remember to give positive rating, as that is the only way I am compensated for my time and effort