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The guzzle 6 adapter is listed as a require-dev package in composer.json: https://gerrit.wikimedia.org/g/mediawiki/extensions/Mailgun/+/9ba8e0d5a86936473c804fe93997bea8048ae506/composer.json#11

But it is used in "production" code. https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/Mailgun/+/master/MailgunHooks.php#46

This results in the following error when creating accounts with the extension enabled.

[803b5fd8f6b4fed2c4ed6c32] /w/index.php?title=Special:CreateAccount&returnto=Special:SpecialPages Error from line 46 of /var/www/html/extensions/Mailgun/MailgunHooks.php: Class 'Http\Adapter\Guzzle6\Client' not found

The fix is to move php-http/guzzle6-adapter from require-dev to require in the composer.json file.