Hi,
According to your description, I understand that attachment has been removed which contain .crt file, after Exchange Edge server reinstalled.
By default, the Attachment Filter agent is enabled on the computer that has the Edge Transport server role installed.
Please run "Get-AttachmentFilterEntry | Format-List" on Edge server to view a complete list of file name extensions and content types. If .crt file exist, please run "Remove-AttachmentFilterEntry FileName:*.crt" to remove.
Also, you can check the configuration for transport rule:
Get-TransportRule | where {$_.Description -like "*attachment*"} | FL name,*description*
However, .crt file may potentially be infected by malware, please careful before remove it from attachment agent and transport rule.
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Allen Wang
TechNet Community Support
