For optimal deliver-ability it is recommended to use only one email service at a time per domain.
Our Exchange hosting service can be configured in such a way that any email sent to accounts which are not found in the Exchange mail system (but which are still part of your domain) will pass through to an alternate mail hosting service, such as email accounts configured in our shared Plesk hosting.
However, our shared Plesk hosting email service always defaults to local mail delivery for optimal security. As a result of this, messages being sent from a Plesk hosted mail account will likely not reach those hosted in the Exchange system and will bounce instead.
That said, if you use another external mail system that 100% supports split domain mail accounts, then you can indeed use our Exchange hosting for just one or two mail accounts while retaining the use of your 3rd party mail hosting service for others.
But please note that we will not provide support for this configuration as any issues that arise as a result of it are strictly the responsibility of your external mail hosting provider.
An alternative to splitting your mail hosting on one domain is to use two separate domains or a subdomain. For example, you could use this configuration:
john@mydomain.com - Exchange hosting service
jenny@mail.mydomain.com - Other hosting service
francine@mydomain.net - Other hosting service
Which avoids the deliverability issues by allowing you to configure the DNS settings for each of the domains and subdomains separately.