How to recover after being attack and send out spam?

hi all, 3 email account that within our box has been hacked ( suspected using the default easy to guess password ) and send out tons of spam email, I have disable the 3 account and clear out the queue already, now I would like to find out what can be done to recover and to prevent happen again.

  1. is there any way to monitor alert like if there are suddenly spike of sending email there will be some notification ?
  2. we been black listed by sorbs, the only way is send in support request to delist right ?
  3. can’t send email to any gmail user with a reject message like this this message is likely suspicious due to the very low reputation of 550-5.7.1 the sending domain, what can we do to fix this ?