Use SquirrelMail program to send out message and encounter the below error:
Requested action not taken: mailbox unavailable
Server replied: 550 Your server announces itself (18.104.22.168) with a plain IP address
which is in breach of RFC2821.
ORas shown in the below screen shot:
The error is due to we have enable a feature EHLO/HELO protection on our Linux mail server. You may refer to the EHLO/HELO protection article at https://support.exabytes.com/KB/a1855/exim-heloehlo-protections.aspx.Due to this matter, you are no longer able to use IP address to send email using Squirrel Mail webmail due to Squirrel Mail configuration.
HOWTO ENCOUNTER THE ERROR:
For our user:
You may encounter the error when you access the Squirrel Mail using the below URL:
http://IPaddress:2095 or http://IPaddress/webmail
You may encounter the error when you try to access the Squirrel Mail using your client hosting account through WHM Control panel.
To avoid encountering the particular error message, please do not use IPaddress to access the Squirrel Mail. Please use your domain name toaccess Squirrel Mail webmail. E.g.: http://domainname:2095 or http://domainname/webmail.
If you would like to use IP address to access your webmail, I suggest that you may consider using Horde Mail instead of using SquirrelMail.