Hi,
In my case, I had two issues.
1. Being that our hosting company had local delivery enabled. This meant that due to the domain the server sending to was the same as the current it was sending the email to address only on that server. Therefore ask your hosting company to disable this plugin.
2. The email hosting company had a spam filter. Therefore we needed to ask them to whitelist our server. I would email your email hosting company to see if they can do this for yourself.
I hope this helps.
Thanks,
Sam
