How to sync Trash and Sent folder on iPhone8 to IMAP server?

My boss has a new iPhone8. I've set him up to access the IMAP server at the office. Inbox works just fine and he can send messages. The problem is that the Sent and Trash folder on the iPhone do no synchronize with the server. Apparently, this is not an uncommon problem since I've found lots of posts about it. However, none of the instructions I've found so far seem to work. How do I do this?

EDIT:
The mail server is Dovecot IMAP. The iPhone is using the normal mail facility, nothing special or 3rd party. The selected server setup is 'Other'. Inbox synchronization works. Messages deleted on the iPhone, and sent messages are not synchronized with the corresponding folders on the IMAP server.
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MarkAsked:
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Kyle SantosQuality AssuranceCommented:
Need version info

What server are you using and what app on the phone?  iPhone Mail app, Outlook, Gmail or ?  Exchange?
bbaoIT ConsultantCommented:
are your able to access the mailbox via web? if you can't see the two folders via IMAP from your iOS device, they needs to be enabled or subscribed from the server side.
MarkAuthor Commented:
The mailbox is accessible from webmail (RoundCube) and from in-office mail client (Outlook). On the iPhone there are Trash and Sent folders, but these do not get synchronized with the IMAP folders. Is there not a way to map these to specific folders on the IMAP server?
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Eoin OSullivanConsultantCommented:
I use DOVECOT with the iOS Mail client and it works fine

On the iPhone open the SETTINGS : Accounts & Passwords : EMAIL ACCOUNT and go into ADVANCED

There you can map the Drafts, Sent, Trash etc .. to the folders you want .. you need to pick the "On The Server" folders .. you've probably got the local version setup
MarkAuthor Commented:
Eoin OSullivan: Thanks. First, a clarification. Sent and Deleted messages are syncing to the IMAP server, but to the folders as named on the iPhone, not the user's IMAP configured folders, For example, the iPhone has a "Sent Messages" folder, and they are going into a folder named "Sent Messages" on the IMAP server. The user wants them to go to "SENT", which is where his Outlook sends them.

Back to your suggestion. Tried that. Went to Settings > Accounts & Passwords > selected the email account > Advanced. Selected the iPhone local "Sent Messages" folder, then "on the Server" folder list, selected the desired folder, "SENT". There was no "Save" button visible, so I assumed it just did it. However, messages continue to go to the "Sent Messages" folder. When going back to "Settings ..." the folder mapping is gone. I tried restarting the iPhone, didn't help.

What are we doing wrong?
Eoin OSullivanConsultantCommented:
As you backup from the Advanced screen did you press DONE to save the settings?
https://www.inmotionhosting.com/support/email/iphone/change-mailbox-behavior-for-an-ios-11-mail-account

I usually do something else like change the REMOVE settings on the same screen for DELETED MESSAGES to see if that makes a difference

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MarkAuthor Commented:
Eoin OSullivan:
As you backup from the Advanced screen did you press DONE to save the settings?
Yup, that did it! Two of us were looking at the screen during this process and I guess we missed the DONE button when backing out.
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