Invalid email subject header on long topic titles

After a recent upgrade to phpBB version 3.3.5, Google began rejecting emails due to a missing 'From:' header. An investigation revealed the 'From:' header was not missing. Instead, the 'Subject:' header was incorrectly formed causing the 'From:' header to be misread. So far I can confirm this error only occurs when sending an email with a subject length of 75 or more characters bug after testing .

Steps to reproduce

1. Create topic with long title such as This topic title is sixty five letters long and will not process.
2. Subscribe to topic
3. Await replies.
4. An email will be sent with a malformed subject line.