Just paid to have email added to my website through homestead.com

I have sent a total of nine emails since adding this account (used for business) and today, received this bounce-back error message less than an hour after sending this email:

This is the mail system at host smtp9.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.

The mail system

<xyz@abc.com>: abc.com

First, why did this not send? Second, why am I only given this version (above) while logged into my Zimbra/homestead account via laptop, but when I check this same error email from an Iphone, it gives me additional header information that includes Action: Failed Status and Diagnostic Code: X-Postflix, among other things?

Is this a Zimbra problem or a homestead problem???

FYI...I signed up for this account because I was not RECEIVING emails using mail.com....now, I'm not able to SEND emails using Zimbra...Any way to prevent this from happening again?