Add service/mail header and recover from service panic

[heilmela]

Description

Used Postmark with service/mail. Postmark uses the X-PM-Message-Stream header to select a message stream via header. Currently its not possible to set headers.
Furthermore the service/mail did panic when encountering some missing TLS port issues, which leads to waiting for the service routines call to hang until context deadline, which might be undesirable in some cases.

I would suggest adding the recover and error handle for the corresponding service. I know recovering is debatable but the hanging because of one service seems undesirable.

@nikoksr wdyt ?

How Has This Been Tested?

• Added a test for the new addHeader
• setup postmark transactional stream
• send some emails

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.