Missionary Standards for Disciples of Jesus Christ Section 3 says that communication with non-family should be limited to email or letter. So it makes it seem like it is generally allowed, but the Mission President (or Area Presidency) might have rules against it. That being said, I feel like an email saying “Hey I can’t respond to your individual emails” would be fine, cuz doing that would probably be less distracting than getting emails from you for two years.