Add: Ability to specify delivery status notification

[Herz3h]

for issue #86

[xhit]

Hi @Herz3h

Thanks for the PR.

I see some code that will broke current implementations here, and I want more simple implementation from dev, I will make changes here, I will let you know for test

Thanks.

[xhit]

Done, please test and let me know, I update the readme in the change, use it as reference.

[codecov-commenter]

Codecov Report

Merging #87 (7205796) into master (47471be) will decrease coverage by 1.18%.
The diff coverage is 22.91%.

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@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   53.81%   52.63%   -1.18%     
==========================================
  Files           7        7              
  Lines        1245     1290      +45     
==========================================
+ Hits          670      679       +9     
- Misses        508      542      +34     
- Partials       67       69       +2     

Flag	Coverage Δ
unittests	52.63% <22.91%> (-1.18%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
email.go	33.17% <19.56%> (-1.00%)	⬇️
smtp.go	77.72% <100.00%> (ø)

[Herz3h]

Nice refactor !

I have tried it, it's working as expected 👍