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Are you sure the recipient's domain are valid? &nbsp;Check and verify the MX using this tool. &nbsp;Check a few of them.<br />
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<a href="https://mxtoolbox.com/" rel="ugc">https://mxtoolbox.com/</a>
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We attempted to send a large amount of email out to our customers last night through our Exchange 2016 server. Everything went well except for a single queue, comcast.net. It sent the majority of the email out, however it is now stuck with the attached image. We have already restarted the transport service as an attempt to correct the issue. Any advice would be greatly appreciated. <br />
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<a href="https://filedb.experts-exchange.com/incoming/2020/03_w12/1447332/2020-03-19_11-46-29.jpg" target="_blank" title="Exchange Error Message (21 KB)"><img alt="Exchange Error Message" class="file-block lazyload" style="max-width: 638px;" data-src="https://filedb.experts-exchange.com/incoming/2020/03_w12/1447332/2020-03-19_11-46-29.jpg" /></a>
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2020-03-19_11-46-29.jpg

We attempted to send a large amount of email out to our customers last night through our Exchange 2016 server. Everything went well except for a single queue, comcast.net. It sent the majority of the email out, however it is now stuck with the attached image. We have already restarted the transport service as an attempt to correct the issue. Any advice would be greatly appreciated. 



Exchange Queue Stuck

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

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.