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Is the citrix server using an Exchange server or are they pointing directly to the ISP's smtp server? &nbsp;
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Is the citrix server using an Exchange server or are they pointing directly to the ISP's smtp server?  

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You are correct. &nbsp;My mailserver flagged it as spam because of that. &nbsp;I whitelisted them for now and alerted them of the issue. &nbsp;Thanks.
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You are correct.  My mailserver flagged it as spam because of that.  I whitelisted them for now and alerted them of the issue.  Thanks.

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Ok, I have a customer that I host e-mail and web for. &nbsp;They have three domains they have e-mail for that I host on my mail server (using hmailserver). &nbsp;Each domains MX record points to a third party SPAM filter (MXLogic). &nbsp;They can send/receive mail just fine. &nbsp;However.....<br />
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They also have e-mail accounts they use from one of their customers. &nbsp;They make citrix connection to their other e-mail and try sending e-mail from their to one of the accounts I host and they get a 550 Validating Sender failure message. &nbsp;I tried googling this but there is virtually nothing out there. &nbsp;Any ideas?
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Ok, I have a customer that I host e-mail and web for.  They have three domains they have e-mail for that I host on my mail server (using hmailserver).  Each domains MX record points to a third party SPAM filter (MXLogic).  They can send/receive mail just fine.  However.....

They also have e-mail accounts they use from one of their customers.  They make citrix connection to their other e-mail and try sending e-mail from their to one of the accounts I host and they get a 550 Validating Sender failure message.  I tried googling this but there is virtually nothing out there.  Any ideas?

550 Validating Sender

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

An email client, email reader or more formally mail user agent (MUA) is a computer program used to access and manage a user's email. A web application that provides message management, composition, and reception functions is sometimes also considered an email client, but more commonly referred to as webmail.

Email Software

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.