chore: improve courier logging

[aeneasr]

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 77.08333% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 78.11%. Comparing base (83792ef) to head (ee8f02f).
Report is 3 commits behind head on master.

Files	Patch %	Lines
courier/smtp_channel.go	58.82%	6 Missing and 1 partial ⚠️
courier/courier_dispatcher.go	86.36%	3 Missing ⚠️
courier/http_channel.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3943      +/-   ##
==========================================
+ Coverage   78.04%   78.11%   +0.07%     
==========================================
  Files         362      362              
  Lines       25342    25324      -18     
==========================================
+ Hits        19777    19783       +6     
+ Misses       4048     4024      -24     
  Partials     1517     1517              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[hperl]

LGTM!