Windows 2016 smtp relay server for Office 365

We build a windows 2016 SMTP relay server for office 365. All the messages passing through SMTP relay is working fine but we have some messages stuck in the SMTP relay queue and those messages will not leave from the queue, it fails after 12 hours. Is there any setting in IIS causing this? Any information regarding this is greatly appreciated.
Sam HaqueSr EngineerAsked:
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Casey WeaverManaged Services Windows Engineer IIICommented:
First things first, enable some logging and then post that information. It can be any range of issues, but most often I see an incorrectly configured SPF record so that some email providers distrust email coming from your relay.
Sam HaqueSr EngineerAuthor Commented:
We are getting the email from our relay without any issues to any domain. The only problem we are facing when our automated program attaches the document and sent an email via the SMTP relay. All of those emails shows on SMTPQueue folder relay server's queue folder.
Sam HaqueSr EngineerAuthor Commented:
It could be ISP issues. It seems to be working with the same configuration on different ISP.

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Sam HaqueSr EngineerAuthor Commented:
We set up another SMTP relay which is pointing to different ISP provider and it worked fine.
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