Suspend Outgoing mail for single mail account

That's likely what's done now, as changing the user's password has long been the standing solution (beyond account suspension, that is). Depending on the host, they may be prevented from changing the user's password for one reason or another.

Interestingly, I can think of a few other reasons I might want to suspend the email account, rather than change the account's password. The first one for me is if you aren't sure if the email they're sending is legitimate or not. In that case you may want to suspend the account in order to buy you time to contact the customer without changing the password (and forcing them to update it on any/all devices they're using for email)

There are others that don't center on account compromise, but rather customer contacts. For example, most of the time SMBs might not notice their site go down, but would notice if they suddenly couldn't send email. You could make suspension of just the email accounts the first step in your account suspension process.

for me this is important, if we can suspend the outgoing we still have

the ability to notify the customer, and they can receive our suspension

notice. as incoming isnt suspended

I agree with oempire. Often time a client only has one email account. So, to suspend his outgoing mail will allow him to receive our support emails, indicating why it was suspended and how to take action.