You should add a simple option to specify mail server hostname when autodiscover is enabled on domain level...

example:
main.domain -> Web
mail.main.domain ->Mail server (this one should be in autodiscover xml response for incoming/outgoing hostname )

Is it currently possible to set only one mailbox to forward the mail for non-existent users in Domains > example.com > Mail > Mail Settings > Forward to address.
Please allow to add more than one email address for this setting.

By current design of Plesk SSL certificates selected to secure webmail in Plesk doesn't secure IceWarp webmail URL.

Consider adding this functionality.

It'd be good to be able to migrate Contact & Calendar from cPanel to Plesk.

Add mail autodiscover functionality for iOS/OS & android mail clients please.

Having a built-in REST API for IMAP, either similar to or compatible with the Gmail API would be amazing, especially if you could limit user rights in Plesk so that you could have domain specific access, or read-only access etc. The imapapi linked to nodemailer is a huge step forward but integration with Plesk would be ideal since security is paramount for an API that can access mail. Obviously this is a long-term request.

By default, the Mail Audodiscover function does not set the Root folder path to "INBOX" in Outlook, which results in an odd directory structure:

Inbox
Drafts
Sent
Trash
Spam
Outbox

Please provide a way to configure this without having to manually edit the mail server configuration files.

Please make Plesk Premium Email available as a PWA (Progressive web apps).

So that we could "install" Roundcube as a PWA on our Phones.

Seeing as the current implementation does not work with Outlook (and a lot of people use Outlook), it would make sense to remove the digest-md5 option from dovecot and postfix configs. These options keep coming back on a Plesk update so it would better to just remove them completely.

https://support.plesk.com/hc/en-us/articles/115001001933-Cannot-set-up-Plesk-mail-IMAP-account-in-Outlook-SASL-DIGEST-MD5-authentication-failed-authentication-failure
https://support.plesk.com/hc/en-us/articles/213917625-Outlook-fails-to-send-an-email-SASL-authentication-failure-On-Plesk-for-Linux

In case some mailboxes send spam on behalf of my customers, I would like to have an ability to block their mailbox from sending emails to particular domain or mailbox that exists externally

Currently, Plesk has an option to enable mail forwarding for particular mailboxes of the domain; and it is also possible to forward all incoming domain's mail to a single mailbox.

Please allow activation of mail forwarding to another domain for all existing mailboxes at once.

To allow one click mail forwarding as follows:
mail for user1@domain.com redirected to user1@domain.net,
mail for user2@domain.com redirected to user2@domain.net, etc.

Example:
Malware has created a lot of spam that was sent from the domain example.com and the mail queue is full.
I would like to have the option to find all the emails from that domain in the mail queue and remove them with one click instead of obtaining the mailqueueIDs and trying to push them to "postsuper -d -".

To make it simple, it's a filter (e.g. sender contains example.com) + option to remove.

Would be good to be able to manage SMTPUFT8 option over Plesk so that it is not reverted after Plesk Updates and also to remove the need to manage this option over the Postfix config directly.

More information:
https://talk.plesk.com/threads/smtputf8-is-required-but-was-not-offered-by-host.351377/
http://www.postfix.org/SMTPUTF8_README.html

Add the ability to download/upload a list of the domains from Whitelist of Blacklist (Tools&Settings > Mail tab) as a text file so it is possible to add all the required domains at once.
The following Uservoices matching this feature were already created serveral times:
https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/15998113-server-wide-e-mail-settings-blacklist
https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/17805055-download-and-upload-for-the-email-blacklist-in-ord
https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/6345455-mail-server-settings-bulk-emails

Add an option to set a delay to the Postfix auto-reply. Currently, it sents instantly.

Not sure how can this be implemented but mailboxes getting full from messages(spam) that gmail does not download

It would be nice if there is a way to auto clean undownloaded messages after a while