EMAIL - If I accidentally mark email as spam, can I reverse it?

Yes - But only while the spam is in the Junk-Email folder.

Go to the Junk E-Mail folder

Select the email that you wish to be considered as valid email. Then select Unmark as Spam from the drop down list.

Press the "Do Action" button,.
to remove the email from the Junk E-mail folder.

If you perform this, the mail server will consider the email as ham (valid email) as such it will add the email address to the trusted domains box in Settings | My Spam Filtering.