Allow relaxed localpart size verification

[jcbf]

Mitigation for #52

[codecov]

Codecov Report

Merging #53 into master will decrease coverage by 0.26%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   87.25%   86.98%   -0.27%     
==========================================
  Files           1        1              
  Lines         502      507       +5     
==========================================
+ Hits          438      441       +3     
- Misses         64       66       +2

Impacted Files	Coverage Δ
smf-spf.c	86.98% <66.66%> (-0.27%)	⬇️

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[coveralls]

Coverage decreased (-0.3%) to 87.135% when pulling 22a144e on jcbf/relaxed_localpart into 0dc862a on master.