emails constantly rejected by recipients who I have emailed for years. Ruining my business

Continue get an error when sending emails from outlook. This does not happen every time but more times than not. This is very frustrating, as we use Homestead for our business. Have reset up emails in Outlook 2007 and the test work but yet emails continue to be rejected by the recipient, whom I have emailed many times over the past few years with no issues... Below is just 1 of the errors we receive.
This is the mail system at host smtp2.homesteadmail.com.

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your own text from the attached returned message.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your own text from the attached returned message.

The mail system

: host 74.220.195.20[74.220.195.20] said:
552 5.2.0 Spam Message Rejected - ERR018 (in reply to end of DATA command)