Denial of service - CiviCRM Fetch Bounces scheduled job will fail to process any emails if a single email is sent to the bounce mailbox with an invalid returnPath
CiviCRM bounce processing will fail to process any emails if a single email ("Denial of service email") is sent to the bounce mailbox with an invalid returnPath.
CiviCRM will repeatedly try and fail to process the "Denial of service email" causing all other emails to remain unprocessed. CiviCRM mail reports are therefore incorrect and ultimately the CiviCRM Fetch Bounces scheduled job fails to perform the job it was designed to do.
Example invalid returnPaths as observed in production:
x2108-arfsfufwyfgkvni2x7ww5wvac3eifc2wx3vr1fcwbbysieqppis4ytnyip09xcohck9s1vrm77nbg4xh431tthdsmtmfvfpa@rspf.mxthunderstruck.net
SRS0=3Ypr33=QM=crm.nothappyjohn.org.au=bounce+b.25211.6039846.eb7e7e8ba282f9d0@airquotes.com.au
This problem has been highlighted by the introduction of "Scheduled Job Failures" feature which raises the visibility of this type of problem.
Bounce mailboxes have been observed with 10k to 50k unprocessed emails, depending on the time when the "Denial of service email" was received.
Agileware Ref: CIVICRM-1970