I noticed a post in 2018 about my same issue. I am using siteground with "Localdir" (the other posting used SMTP) because our email blasts are small volumes. The emails go out ok, but the civicrm email reports can't find bounces, and clickthroughs ? A siteground support person spent 2 hours "chatting" online trying to help me, but no success. Siteground uses "dovecot" email and all civicrm emails create folder-level addresses like ".bounces.ignored" and "bounces.processed" with three folders under each - "cur, new, tmp". Not sure how to fix/patch this to work ? I am using drupal 7.69 with civicrm 5.22.
I presume that you are talking about bulk mailings. I have got this working for small volumes hosted on Siteground. I had a lot of problems originally with subfolders though can't remember the details. My solution was as follows.
Outbound use mail() and in Administer>>CiviMail>>Mail Setting, set Mailer Batch Limit to 100 and Mailer Cron Job Limit to 1. With the Send Scheduled Mailings cron job running every half hour this throttles the outbound mail within SiteGround's limits.
Inbound i set up an account on GMail to handle the bounces so bounce processing account (Adminster>>CiviMail>>Mail Accounts) was set up using IMAP to GMail and address email@example.com and localpart return+ and email domain of return.example.org. I then forwarded @return.example.org to firstname.lastname@example.org. The GMail account needs some special settings as described in https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/inbound/#return-channel-google-apps. All rather complicated but nothing else I tried worked.
Hope that helps.