Affected version(s):
OX App Suite 7.10.0 and above
Dovecot 2.3.7.x and below
Problem
Mail previews are not shown or just in rare cases in the App Suite Mail UI even if enabled via configuration found here.
Solution
App Suite requests previews from supported IMAP servers providing this capability via the LAZY priority modifier.
This modifier instructs the IMAP server to only return the requested data if it is available more or less instantly. Specifically for Dovecot that means the body.snippet needs to be already in the index. It is not by default in the indexed fields though and up to Dovecot version 2.3.7.x it was also not put in the index on such kind of requests. This means that even multiple requests and new mails typically do not make the previews appear.
Since Dovecot 2.3.8 those requests will teach the autolearning index to index this data for new mails. Snippet of the changelog:
When client issues FETCH PREVIEW (LAZY=FUZZY) command, the caching decisions should be updated so that newly saved mails will have the preview cached.
To achieve the same thing the index field can be listed in the mail_always_cache_fields as in this example:
mail_always_cache_fields = flags date.received size.virtual imap.bodystructure mime.parts hdr.importance hdr.x-priority hdr.references body.snippet
doveadm force-resync may be used to force reindexing for already existing mails but please keep in mind that this might be a very expensive operation depending on mailbox sizes etc.
Related articles
https://documentation.open-xchange.com/7.10.2/middleware/mail/mail_text_previews.html
https://tools.ietf.org/html/draft-ietf-extra-imap-fetch-preview-06