All SMTP related issues are high priority. Report these here or at simplesmtp.
"Sendmail" transport related issues are low priority. If you want something to get fixed, making a pull request instead of filing an issue helps a lot to speed things up. Most probably the issue is related with a specific sendmail implementation and I'm not able to test every sendmail clone out there.
When making a pull request try not to introduce anything that is not backwards compatible.
Use 4 spaces instead of tabs. Commas last. Use double quotes instead of single quotes where possible.