This suggests you don't have any SPF record for your domain. If it is your domain and you have control of the DNS record, then it's usually up to the owner of the domain to set the DNS records up and add the SPF record.
SPF helps identify SPAM where someone is spoofing emails from your email address or domain, as it will be seen they are sending email from a server that isn't in the list of servers you use. Usually SPF is used to score the email for the likelihood it's SPAM, so if you have no SPF record you lose points for that, and if you have an SPF record you lose even more points if the email originates from a server not on the SPF list.
The recipient of your email (the email server) will do a DNS lookup and try to find the SPF record, so yes you are saying this is all the IP addresses I legitimately use for emails from mydomain.co.uk. If you are sending your email via Zen servers, then their SPF settings should also be the ones you require.
Zen's SPF record is:
v=spf1 include:_spf.zen.co.uk include:_spfcoremail.zen.co.uk include:_spf.salesforce.com include:aspmx.pardot.com include:servers.mcsv.net include:amazonses.com a:smtp.insmartcloud.com ~all
This is basically all the servers Zen uses to send email, it looks like it includes servers they do mass emailing from, you probably don't need these mass emailing servers but it will do no harm just to copy the whole thing for the sake of setting something up, so here are a few pointers, this assumes you do send your email via Zen's SMTP servers
1) Get a base mark of your domain, go to https://mxtoolbox.com/SuperTool.aspx
and enter your domain name and then select SPF Record Lookup, it will most likely come back saying there isn't one.
2) Go to whoever does your DNS on your domain name, they should give you options to add DNS settings, you need to add a TXT record, simply copy the one above that Zen uses.
3) Now run step one again, it should come back with the TXT record you entered, if not, you may need to leave it a while for the settings to go live.
4) Once you can see the SPF record coming back in step 1, try a test email again to Gmail, with any luck it now says SPF passed.
Hope that helps.
Edited by PhilipD (Wed 29-May-19 07:53:08)