If you are experiencing issues with your contacts not receiving emails from your domain, try the following;
1. Go to your website, and at the end of your Domain Name, add a forward slash and type in "webmail" after it. It should look something like this:
https://mydomainname.com/webmail
2. Alternatively, if that does not work, you can use the portal command to get to the same location - your email server. By adding a semicolon at the end of your domain name followed by the number 2096. It should look like this.
https://mydomainname.com:2096.
Use the email address you are having sending issues as the username, and the password that was issued to you at the time of set up.
Once you have logged in, Choose "Roundcube" as your email client. This will then take you to your Email Server where you should see your emails as per how you would see them in your desktop email client.
Locate an email that was sent to you by the email address you are having issues with. Right click on the Senders Name or Email address, and choose from the sub-menu White List this address.
Repeat this step for every email address you are having issues with. The common email server that causes either replies to go to their SPAM or for the "Undelivered" Message bounce back response are Hotmail and Outlook (Microsoft) email extensions e.g. @hotmail.com and @outlook.com.
It may also help if your contact does the same, by adding your email address to their Trusted List.