I have finally managed to send emails to recipients but not all of emails are being delivered to all addresses. Using the source of a successfully delivered email I suspect the following: Authentication-Results: ukmail.iss.as9143.net; spf=softfail (22.214.171.124;mywebsite.com); dkim=pass header.d=mywebsite.com; dmarc=pass header.from=mywebsite.com (p=none sp=none dis=pass); "spf = softfail" SoftFail means the IP address may or may not be authorized to send from the domain. The mailbox provider will likely mark the message as suspicious, however, they will still accept it. A softfail does not necessarily cause deliverability problems by itself because mailbox providers rely on other data points to make a filtering decision. Ensure your SPF record is up to date with your sending server or IP address information. Checking SPF record via DNS: TXT v=spf1 +mx +a +ip4:126.96.36.199 ~all Analysing SPF: "all SoftFail Always matches. It goes at the end of your record." SPF Failed for IP - 188.8.131.52 184.108.40.206 = Zwolle Ziggo B.v. based in Holland. Also "DKIM Signature Verified : Signature Did Not Verify" Public and private key : "The receiving email system applies local policies based on the results of the signature test. For example, the message might be deleted if the signature does not match." Not sure where to go from here to fix this issue.