No suggestion, but a critical, urgent, overdue feature request, as described in many "suggestions" since years:
Secure
mail.customerdomain.tld
webmail.customerdomain.tld
with Let´s Encrypt certificate without having to set the DNS A Record for
customerdomain.tld
to the mailserver IP.
And withouth having WAF disabled (SSL It 1.20 is being blocked bei Comodo (free) and Atomic (free) rules, other not tested yet).

Add functionality to import csv files to create multiple e-mail accounts from a list in Plesk Panel

Many of our clients have requested the ability to manage their email configuration options through webmail, rather than via Plesk. I'd like the option to allow mail users to manage their account themselves via a Plesk Roundcube plugin that provides the following options:

1. Setting up an Autoresponder
2. Configuring forwarding / redirects
3. Changing their password

Others that would be handy as well:

A. Managing server-side mail filtering (whenever this becomes available in Plesk at all)
B. Spamassassin training

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

I would like to enable and configure fetchmail on mailbox level. Main reason is easy migration for customers to new hosting providers including moving mails from existing mail servers.

"fetchmail" is already installed together with PLESK, needs just to get properly configured.

Don't send Auto-Reply when message detected as spam

When a mail account has Auto-Reply and Spamassassin enabled, the Auto-Reply function should read the x-spam header and, if the incoming message was found to be spam, the automatic response should not be sent.

At the moment I often see Automatic replies stuck in the outgoing mail queue because they're being sent to spammers. This is silly!

With option to disable or enable.

Can be installed manually but all guide are now old and nothing for Onyx.

This worked really well for me on 12.5.

If i add a new domain in a subscription, greylisting is automaticly activated, just because it's enabled on the server.

It would be much better if customers have to activate greylisting manually in their settings, like the spamfilter.

A workaround with Event Manager is possible but a bit inconvenient.

It seems difficult to manually install a mobile skin on Roundcube, and if it should succeed the plesk would overwrite the config made on updates. The default skin provided with plesk should support mobile devices, as this is an important feature nowadays.

Add a setting to disable mail forwarding to certain domains in Panel for global configuration so that on user side wont be allowed to put the certain domains in the Forwarding Email in the Plesk Panel. Thank you.

Currently, there is no way to permanently change the MSMTP config in /etc/msmtprc and the Plesk options in "Mail > External SMTP Server" are not enough for Gmail and Zoho to even work.

Gmail problem:
While sending you get
errormsg='the server sent an empty reply'
Fix: add "tls_starttls off" to /etc/msmtprc
Zoho problem:
https://talk.plesk.com/threads/its-not-possible-to-send-notification-emails-from-plesk-using-and-external-smtp-msmtp.344922/#post-836533

Please provide for us a way to change the config template or include additional configs, like we have for e.g. proftpd in /etc/proftpd.d/

Related suggestions:
https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/17680498-msmtp-change-sender

I would like to be able to create own e-mail addresses for Domain Aliases. Let's say i have example.com as domain and add example.net as alias, now i would like to create e-mail adresses for example.net (alias domain) only which are NOT available on example.com and vice versa.

Required External/Remote mail server like mailenable, smartermail with different server.

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

There is a Mail Importing feature in Site Import extension, but it is possible only to migrate emails one by one. It would be great if there is an ability to migrate a couple of mailboxes at once.