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Description of problem:
The postfix-3.5.8-6.el8.x86_64.rpm file in the CentOS stream repository is corrupt.

Version-Release number of selected component (if applicable):
3.5.8-6

How reproducible: consistently

Steps to Reproduce:
1. Download the file from here: curl -O http://mirror.centos.org/centos/8-stream/BaseOS/x86_64/os/Packages/postfix-3.5.8-6.el8.x86_64.rpm
2. Try to install itt (rpm -ivh postfix-3.5.8-6.el8.x86_64.rpm)
3. Behold the error message: 
  error: postfix-3.5.8-6.el8.x86_64.rpm: signature hdr data: BAD, no. of bytes(77508) out of range
  error: postfix-3.5.8-6.el8.x86_64.rpm cannot be installe

Actual results: Errors

Expected results: No errors

Additional info:
Previous release is okay

If you download the file from https://composes.stream.centos.org/stream-8/production/CentOS-Stream-8-20230622.0/compose/BaseOS/x86_64/os/Packages/postfix-3.5.8-6.el8.x86_64.rpm does it have the same issue?

Yes, it has the same issue.
But it's no wonder as it has the same MD5 hash.
https://composes.stream.centos.org/ 8d3f1feb8ddbd14099fc8991904a6f5f  postfix-3.5.8-6.el8.x86_64.rpm
https://mirror.centos.org/          8d3f1feb8ddbd14099fc8991904a6f5f  postfix-3.5.8-6.el8.x86_64.rpm

postfix-3.5.8-4.el8.x86_64.rpm (notice the release: 4) has no problem.

This is fixed in both composes and mirrors — managed to install postfix-3.5.8-6.el8.x86_64.rpm (release 6) just fine.

It seems that it was actually caused by RPM. 
4.11.3-45 error
4.11.3-48 works