An OnDemand user has noticed that some staff replies are not reaching the recipients' inbox even though there aren't any errors reported in Kayako. Moreover, these emails are not being flagged as spam or quarantined and are simply not arriving.
One possible reason for this behavior is having an attachment as part of the reply that exceeds the email size limit defined in Admin CP > Settings > Email Parser > Settings. Outbound emails exceeding this limit will not be generated and won't log an error.
As indicated in Email Attachment Size Limits in Kayako Classic, the default email size limit for On-Demand instances is 10MB and should not be increased.
Therefore, to prevent this issue from occurring it is recommended to compress attachments as to not exceed the email size limit or to use other file sharing methods.
Once the email size/attachment is below the limit, emails should arrive without any issues.