Right now spf hook checks incoming mail for all domains if it is enabled in Tools & Settings > Mail Server Settings.
There is no simple way to disable it for some domains/subscriptions that may not need it present.

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/

Request to when a customer clicks to log in on the webmail it shows the options of webmail platforms he can select.

Example attached:

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

Hello,

Please add the possibility to purchase an extended license for mail servers with Plesk.

It would be helpful if I prefer to separate my mail services from my main hosting.

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.

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.

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

In the email autoreply function, please extend the option to save corresponding texts for various situations (holidays, inventory, illness, sabbatical, etc.) and then select and edit them as needed. Many Thanks.

When creating backup of domain and using SmarterMail, contact lists are not backed up.

Please consider this option so that contacts are also backed up when performing a backup.

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

Request fron all Mailservices

Allow creating a dedicated mailbox storage pool to explicitly preserve free disk space for mail data.

New webmail https://www.getmailbird.com/

Only webmail, no windows mail server

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....

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

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

For our customers I need to be able to use "smith.domain.com" as their Email Username, as opposed to their email address