Many things can cause this to happen, but the message they get back will usually say WHY it’s being returned (or bounced)
Reasons email bounce: (in order of most common to least common)
A – Mistake in Typing Your Email Address
B – Your Mailbox is Full
C – Your account is blocking them due to their account being on a blacklist
D – You are or were on Vacation and had setup an auto-reply but forgot to turn it off
E – Your Email server was temporarily unable to receive which is usually due to maintenance at the time. But this is very rare.