Getting 554 Denied Error when sending an email message

If you get this error it means the message failed permanently, at the receivers server normally it will generate a bounce bounce back to the sender....and will not be re-delivered or re-tried.
example:

554 Denied - Denied for Spam Content

Solution:

If you believe that the message is legitimate and should be delivered, contact the recipient and let them know what you are seeing and request that they Whitelist / Allow your sending address.

Why this happens? 
Although any spam filtering, Firewall or Mail Server can reply with a similar type of error this normally happens when the recipients server is using some type of spam filtering service.

Here are some of the common Spam Filtering service that are know to send back a 554 Denied

MXLogic AKA McAfee

Example Error

host domain.com.inbound15.mxlogic.net[208.65.144.12] said: 554 Denied (Mode:normal) (in reply to end of DATA command)

Resolution/Workaround

For McAfee SaaS Customers, please do one or more of the following:

1. Place the sending domain on the User or Domain-Level allow list in the Control Console.  For instructions on how to do this, please refer to Adding to Domain-level Sender Allow List
see full description here:  https://support.mcafeesaas.com/MCAFEE/_cs/AnswerDetail.aspx?sSessionID&aid=155


Godaddy

554 This IP has been temporarily blocked for attempting to send too many messages containing content judged to be spam by the Internet community. IB110
for the full details click here: https://support.godaddy.com/help/article/3568/dealing-with-email-bouncebacks



TIP: In some cases if you get 554 Denied "RELAY is NOT ALLOWED" then the issue is actually on the sending end normally means that when you try to send the message the sending server didn't trust your message and bounced it back, this normally happens if your IP or Domain is not recognized by the sending server as valid.