Outlook 2016 not using autodiscover XML

Question

So, this will be my last attempt to get some help on this issue. I already made a reddit post about this as well, but so far, no solution.

I'm running an ISPConfig server. It has a plugin for Automail (autoconfig & autodiscover). Autoconfig for Thunderbird is working fine. Uses all the right settings. Autodisover (Outlook 2016 and W10 mail) use the wrong settings.

tldr: Outlook 2016 uses wrong server details (imap/smtp.client.com) instead of returned XML (mail.server.com). I don't understand why.

This image should show the flow for autodisover:

What i have done so far:

1 https://testconnectivity.microsoft.com/ -> Microsoft Office Outlook Connectivity Tests Only warning is: The Microsoft Connectivity Analyzer can only validate the certificate chain using the Root Certificate Update functionality from Windows Update. Your certificate may not be trusted on Windows if the "Update Root Certificates" feature isn't enabled.

2 https://learn.microsoft.com/en-us/outlook/troubleshoot/domain-management/unexpected-autodiscover-behavior -> Used the registry settings to disable all ways autodisover works except the SRV lookup

• PreferLocalXML => 0
• ExcludeHttpRedirect => 1
• ExcludeHttpsAutoDiscoverDomain => 1
• ExcludeHttpsRootDomain => 1
• ExcludeScpLookup => 1
• ExcludeSrvRecord => 0
• ExcludeLastKnownGoodURL=> 1
• ExcludeExplicitO365Endpoint => 1

3 Use the outlook "automatic e-mail configuration test" (while outlook is open (no account configured), ctrl+rightclick the systray icon) and test the account. All the test succeed, only SRV lookup is used and the returned XML is correct with all the right settings (server, ports etc).

The strange thing is that Outlook is still using imap.client.com and smtp.client.com instead of the XML values returend bij the SRV lookup.

The XML returend is:

<?xml version="1.0" encoding="UTF-8"?>
<Autodiscover xmlns="http://schemas.microsoft.com/exchange/autodiscover/responseschema/2006">
  <Response xmlns="http://schemas.microsoft.com/exchange/autodiscover/outlook/responseschema/2006a">
    <Account>
      <AccountType>email</AccountType>
      <Action>settings</Action>
      <Protocol>
        <SSL>on</SSL>
        <Type>IMAP</Type>
        <LoginName>info@client.com</LoginName>
        <Server>mail.server.com</Server>
        <Port>993</Port>
        <SPA>on</SPA>
        <AuthRequired>on</AuthRequired>
        <DomainRequired>on</DomainRequired>
      </Protocol>
      <Protocol>
        <SSL>on</SSL>
        <Type>POP3</Type>
        <LoginName>info@client.com</LoginName>
        <Server>mail.server.com</Server>
        <Port>995</Port>
        <SPA>on</SPA>
        <AuthRequired>on</AuthRequired>
        <DomainRequired>on</DomainRequired>
      </Protocol>
      <Protocol>
        <UsePOPAuth>on</UsePOPAuth>
        <SSL>on</SSL>
        <Type>SMTP</Type>
        <LoginName>info@client.com</LoginName>
        <Server>mail.server.com</Server>
        <Port>465</Port>
        <SPA>on</SPA>
        <AuthRequired>on</AuthRequired>
        <DomainRequired>on</DomainRequired>
      </Protocol>
    </Account>
  </Response>
</Autodiscover>

But what outlook shows when turning on logging is:

{
  "Session ID":"587DD5A1-9F5B-4412-8B0F-060F7070C523"",
  "Client ID":"B34F4741-9DB1-4E80-93AD-3DD5830DD83F"",
  "Profile name":"Outlook"",
  "Accounts in profile":[]",
  "AutoDetect settings":
  {
    "Services":[]",
    "Protocols":[
      {
        "Protocol name":"ExchangeAutodiscover"",
        "Autodiscover":""",
        "Domain":""",
        "Server name":"autodiscover.server.com"",
        "Port":0",
        "Encryption":"Unknown"",
        "Protocol insecure reason":"Unknown"",
        "User name":"info@client.com"",
        "Validated":false
      },
      {
        "Server type":"Incoming"",
        "Protocol name":"IMAP"",
        "Autodiscover":""",
        "Domain":""",
        "Server name":"mail.client.com"",
        "Port":993",
        "Encryption":"SSL"",
        "Protocol insecure reason":"Unknown"",
        "User name":""",
        "Validated":false
      },
      {
        "Server type":"Outgoing"",
        "Protocol name":"SMTP"",
        "Autodiscover":""",
        "Domain":""",
        "Server name":"mail.client.com"",
        "Port":465",
        "Encryption":"SSL"",
        "Protocol insecure reason":"Unknown"",
        "User name":""",
        "Validated":false
      },
      {
        "Server type":"Incoming"",
        "Protocol name":"POP3"",
        "Autodiscover":""",
        "Domain":""",
        "Server name":"mail.client.com"",
        "Port":995",
        "Encryption":"SSL"",
        "Protocol insecure reason":"Unknown"",
        "User name":""",
        "Validated":false
      }]
    }",
    "Account config settings":
    {
      "Account type":"Office365"",
      "Email address":"info@client.com"",
      "User principal name":"info@client.com"",
      "Exchange server resolved":false",
      "Number of protocols found":3",
      "Exchange protocol found":false",
      "SMTP address":""",
      "Legacy DN":""",
      "Internal OWA URL":""",
      "External OWA URL":""",
      "Number of alternate mailboxes":0",
      "Server settings":{}
    }
}