Currently, it is possible to create hourly limits for outgoing mail.
It will be useful to be able to configure it for other timeframes, such as Monthly outgoing mail limits.

The ability to use nginx as mail proxy.

For example, as Postfix does not support SNI, such feature would be useful

https://docs.nginx.com/nginx/admin-guide/mail-proxy/mail-proxy/

Could you please add support for dovecots subaddressing feature?

Some mails could not be received due to the fact the Postfix 2.8 and higher in Plesk uses only TLS encryption, when senders use other encryption methods. Manually updating main.cf directly on the server (as discribed in https://plesk.zendesk.com/hc/en-us/articles/213386889) is not always an option since the main Postfix configuration file may be overwritten by any Plesk update. In order to avoid potential issues with non-delivered mails it is required to make SSL/TLS encryption on port 587 optional, not mandatory.

Please add functionality to configure redirection of single domain webmail.example.com to another URL

Currently, it is possible to apply it using the workaround:

https://support.plesk.com/hc/en-us/articles/360003509474-How-to-redirect-single-domain-webmail-example-com-to-another-URL

Add a feature that allows the configuration of Postfix relay to Exchange for a specific domain.

For example, all outgoing emails from @example.com are relayed to an Exchange/Office 365 server.

The DMARC support in Plesk Onyx 17.5.3 ist incomplete. If an incoming email is rejected due to a DMARC fail there is no mailer daemon generated to inform the sender of this email.

Please add this important feature to DMARC support. Otherwise it would not be usable for most customers.

Currently Plesk Mail certificate only secure POP/SMTP and IMAP connections, would like a feature to encrypt outgoing messages itself.

It will be better if plesk Outgoing Mail Control shows email instead of domain name.

Restrict what attachments are to be filtered by Mail server. Example
All Domain > Block Extension > exe,jar,rar,gzip

It's required to send Plesk notifications using external Mail server at the same time keep local mail server(postfix) working for domains.

Horde already supports GPG encryption and key management. It would increase mail security a lot if it could be configured from plesk.

Hi!
It would be great if the autoresponder may switch on for a whole domain

By default use an existing email created by default as a return path . Return path should be configured in mail tab instead of solution from here:
https://support.plesk.com/hc/en-us/articles/115000419013-What-is-the-return-path-sender-in-PHP-mail-scripts-and-how-to-change-it-

Allow a@example.com only to send mail to example.com

Hello.

Currently Outgoing Mail Limit counts local forwarded messages from existing mail boxes:
https://docs.plesk.com/en-US/onyx/administrator-guide/mail/protection-from-outbound-spam.71349/

Please add functionality to exclude messages forwared to the local mailboxes from counting by Outgoing Mail Limit.

Add event in Plesk that will allow to suspend a domain when outgoing mail limit has been reached.

There is a harcoded delete_interval="30" in /usr/local/psa/admin/sbin/spamtrain. So nearly all my spam just got wiped and SpamAssassin has nothing to train with.

Please make this interval costumizable and deactivable.

Presently, I normally learn that a client has a full mailbox when they call with email issue. I am thinking it would be nice if when I login the Plesk admin control panel, it would be nice if a small area could show if any mailboxes were at 95% or great capacity. Maybe call it Mail Level Monitor. Then if I could decide to up the level, or contact the client to talk about mail client settings & mail management. A proactive approach.

if smartermail is local install you can create a SSL for webmail. This doesn't work with remote smartermail.

I would like plesk to introduce this so some kind of agent is running on the server so Plesk can communicate with IIS on the remote web server to copy SSLs and bindings.