Q1. What is the problem that you are trying to solve?
We have passbolt up and running in our company and we use it regularly. Nowadays, we are required to suffix all our emails with specific company security flags e.g. /aa/, /bb/ etc. otherwise they will not get distributed.
At the moment its unknown/impossible (for me) to configure a suffix for mails, as a consequence all passbolt mails are not distributed to clients at the moment
Q2 - Who is impacted?
The impact is only for those who have similar regulations as our company.
Q3 - Why is it important and/or urgent?
Its only important for us. 
Q4 - What is your proposed solution? (optional)
It would be cool to have an environment variable “EMAIL_SUFFIX” which is basically appended on all email subjects. Nothing more is needed