TL;DR Your email is displayed in grey because it is deactivated and cannot be used for sending. If you cannot activate it with the switch button next to it there are no empty slots on your account to which you could connect it. You can either purchase additional slots » or deactivate one of your currently active email accounts »
Purchasing additional slot
In order to buy an additional slot, go to the Marketplace → Slots tab and click on the green button on the right.
You’ll be redirected to the page where you can choose how many slots you want to purchase and you’ll see how your period payment will change.
In the summary, you will see the difference between how many slots you had (and how much you’ve been paying) and how it will change after confirming your purchase.
Once you have an additional empty slot, simply go to Settings → Email Accounts and hit the switch button next to the inactive email account.
Switching email accounts connected to slot
If you want to avoid additional charges you can always deactivate your email account in order to empty your slot.
Once your email is deactivated you’ll see that the number of slots in use will decrease which means that you have free space for adding (or activating) another email account.
In Agency Panel
You can manage your clients’ email accounts directly from the list of companies. Simply click on the arrow right to their company name and you’ll see emails under their account.
Remember that in order to be able to activate an email account, there needs to be an empty slot on your main Agency account.