Changed logerror to logdebug

[lnedry]

Fixes #
This plugin was logging errors for valid Mailing-list emails.

Changes proposed in this pull request:

• added googlegroups
• changed logerror to logdebug for "mlm start miss". Emails from a yahoogroups list would be logged as an error.

Checklist:

• docs updated
• tests updated
• Changes updated

[codecov]

Codecov Report

Merging #2294 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2294   +/-   ##
======================================
  Coverage    59.5%   59.5%           
======================================
  Files          28      28           
  Lines        6060    6060           
  Branches     1502    1502           
======================================
  Hits         3606    3606           
  Misses       2454    2454

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