An internet search would have given you these results:https://www.startpage.com/do/dsearch?query=spf+best+practice&cat=web&pl=opensearch&language=english

Try Rspamd, it's free, easy to install and configure, lower resource usage than the current ZCS anti-spam solution.

Well, thanks for All the replies and suggestions. That comment is a bit unnecessary, you're making an assumption that people have seen and read your post. We all post here when we have the time and/or an answer to a problem so maybe you were just unlucky. ;) The problem has been solved. For others ...

fsouleymane wrote:Hi,
How to add the IP address of the server in the list of "MTA Trusted Networks"?

Read the wiki article on the subject or search the forums, this question has been asked & answered many times.

As that's an old version of ZCS I'd suggest you upgrade to the most recent version (to keep up with bug and, more importantly, security fixes) and see how you get on with that.

You may get that Zimlet working but as Zimbra Desktop is EOL and no longer supported I'd suggest you install a more up-to-date client and one that can export EML files.

Unless you've modified the output you've posted it would appear you have no Reverse DNS record for your mail server.

You haven't actually mentioned how your Outlook users are connecting to the ZCS server, which port are they actually using?

williaml wrote:Using the zextras suite within 30 days of free trial?

Yes, there's a thirty day trial that you can use.