Impossible to create a wiki account : internal error

[surtsan]

Got a internal error when trying to make a wiki account :
[b4365e2a] /index.php?title=Special:UserLogin&action=submitlogin&type=signup MWException from line 291 of /usr/share/nginx/html/includes/mail/UserMailer.php: PEAR mail package is not installed
Backtrace:
#0 /usr/share/nginx/html/includes/User.php(4260): UserMailer::send(MailAddress, MailAddress, string, string, array)
#1 /usr/share/nginx/html/includes/User.php(4233): User->sendMail(string, string)
#2 /usr/share/nginx/html/includes/specials/SpecialUserlogin.php(437): User->sendConfirmationMail()
#3 /usr/share/nginx/html/includes/specials/SpecialUserlogin.php(333): LoginForm->addNewAccount()
#4 /usr/share/nginx/html/includes/specialpage/SpecialPage.php(384): LoginForm->execute(NULL)
#5 /usr/share/nginx/html/includes/specialpage/SpecialPageFactory.php(553): SpecialPage->run(NULL)
#6 /usr/share/nginx/html/includes/MediaWiki.php(281): SpecialPageFactory::executePath(Title, RequestContext)
#7 /usr/share/nginx/html/includes/MediaWiki.php(714): MediaWiki->performRequest()
#8 /usr/share/nginx/html/includes/MediaWiki.php(508): MediaWiki->main()
#9 /usr/share/nginx/html/index.php(41): MediaWiki->run()
#10 {main}