I suggest
Currently, the whitelist for each mail account is stored as individual lists. In groups (families, small businesses, clubs, etc. ) these lists are time-consuming to maintain. It is easier to maintain these as a matrix. So I suggest to keep the TAB "whitelist", but to link to a single central matrix file.

Please add functionality to forward an email to non-existent user to the sender on the email in example.com > Mail Settings > What to do with mail for non-existent users.

It would be nice if a Preflight check were there. Check SPF Record, Forward and Backward DNS lookup, Send a testmail to mail-tester.com and parse the result to have ideas why own mail are spam....

If you have several ip-addresses on your server it should be possible to choose a different certificate for each ip address.

Allow the Pleask user to specify a mailbox with no (zero) capacity for the purpose of managing email from a 3rd party such as Gmail. This would allow email to be forwarded to Gmail without the exceeded capacity error but also allow Gmail to send mail through the mail domain. Currently this needs to be modified by admin and it's hard to explain each time to the admin.

Currently, there are three buttons in the mail queue interface.
Refresh, Remove, Clear

These are fine when there is Spam in the mail queue but for normal emails, it would be nice to have an additional "Requeue" button.

Now, when I spot an email that shouldn´t be deferred I have to log in over SSH and execute: /usr/sbin/postsuper -r ALL to requeue them.

Just like the "Remove" button works for selected emails the "Requeue" button would work for requeueing selected emails.

The use case. Messages deffered to Yahoo when many messages sent:
421 4.7.0 [TSS04] Messages from IP temporarily deferred due to user complaints

Is it possible to add limit outgoing messages to specific service, for example:

Sent no more than 100 mail to Yahoo per hour

Hello,

At the moment mail server shipped with Plesk is not able to store or handle emails compressed by Gzip on the file system.

Please add functionality to support compresses emails or option to enable email compression.

If you have configured additional delimiters in postfix (/etc/postfix/main.cf) it is not possible to successfully deliver a mail to a mailing list - if that mailing list address contains also one of the symbols configured as delimiter.

The solution is very simple and could be a very generic change in plesk standard config:

In /etc/postfix/master.cf just change the config string for mailman in the end of line from ${nexthop} ${user} ${recipient}
to:
${nexthop} ${mailbox}
This will parse all necessary parameters before the @.

Taken from: http://www.postfix.org/pipe.8.html

Add delete/reject option for addresses and or domains in the blacklist. If I've blacklisted a domain, chances are that I don't want anything from that source.

Additional Administrator Accounts would like to stop receiving certain emails generated by Plesk. Would like to be able to control these.

Recent example of one not needed by one additional admin
"Could not issue/renew Let`s Encrypt certificates for Administrator"

Add to email address mailbox configuration - after mailbox size.

I would like to see the possibility to enable periods ('.') in email folders. Reason for that is that there are some webshops around here using the period as full store name and I would like to keep those emails in a separate folder.

As Google or Microsoft mail service, it can be great if when a user of a mailbox the name of the sender is overwritten by plesk.

Like this, we can provide a better and secur mail server. Mainly for developper or webmail.

Exemple :
I use the mailbox JD@domain.com and on few mailserver (as google), the sender will be JD if we use webmail, php script, etc. But if when we create the mailbox, and we assign the name "John Doe" to this mailbox, the sender will be John Doe.

Thanks for your help

Add the function in Plesk to be able to modify the return path on the autoresponder of MailEnable

If I import mails I enter username and password. These are stored so I can synchronize at any time. But when I wish to delete the credentials, there is no option for it! Username and password are store in clear text somewhere on my provider's server and they cannot be deleted. That is a security failure!

Plesk should be able to create email groups. Most email servers, including the MailEnable that comes with Plesk, implement email accounts, email lists, and email groups in quite different ways. Plesk allows us to create email accounts and mailing lists, but it does not allow us to create email groups. Instead, to get the result that an email group offers, in Plesk you need to create an email account, disable the email box and set up forwarding to addresses that are part of the "group" . This is a little weird.

Hello,

It would be amazing to be possible to create a distribution list via Plesk Premium email without creating a mailbox and forward the emails.

eg. create a example@example.com without a mailbox and forward to a group RH or Sales.

The mail account settings under 'Home > Domains > (domain) > Email Addresses' display the users own domain as mail server. (when clicking the "i" on the right side)
However, the domain on the certificate of the mailserver is usually different, as this is configured under 'Home > Tools & Settings > SSL/TLS Certificates' and applies to all domains and e-mailaccounts on the Plesk server.

This leads to issues when a client accepts the unsafe certificate when setting up his mail client, but this certificate is later changed (as happens more often when using Let's Encrypt certificates).