return failure added to _sent_failed

[Prathm-s]

Fixes #1611

[Gallaecio]

Do you think you will be able to include a test for the change? (I’m not sure we need one here, but they are always a welcome addition)

[codecov]

Codecov Report

Merging #5880 (d73bea9) into master (8045d7e) will decrease coverage by 5.63%.
The diff coverage is 0.00%.

❗ Current head d73bea9 differs from pull request most recent head b83fa60. Consider uploading reports for the commit b83fa60 to get more accurate results

@@            Coverage Diff             @@
##           master    #5880      +/-   ##
==========================================
- Coverage   88.85%   83.23%   -5.63%     
==========================================
  Files         162      162              
  Lines       11057    11058       +1     
  Branches     1801     1799       -2     
==========================================
- Hits         9825     9204     -621     
- Misses        954     1590     +636     
+ Partials      278      264      -14     

Impacted Files	Coverage Δ
scrapy/mail.py	76.66% <0.00%> (-1.99%)	⬇️

... and 17 files with indirect coverage changes

[felipeboffnunes]

@wRAR if you think this is good as it is, lets merge it and close the issue. For me the PR is fair enough.