Use a message header analyser, like Email Header Analyzer, RFC822 Parser - MxToolbox, and posting the redacted result here if the cause of the delay is not obvious; do remember to select the Permanently forget this email header link toward bottom of page when finished with result.
FYI: in webmail select the message, then ☰ > View source and select text from the top to the first blank line; otherwise follow the tutorial here How to Get Email Headers – A Guide from MxToolBox.
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