Попробуйте гем pony. У нас через него русские письма отправляются на ура (и даже китайские).
require 'pony'
Pony.mail(
:to => 'touser@site.com',
:from => 'fromuser@site.com,
:subject => 'mail subject',
:headers => headers,
:body => 'mail content',
:charset => 'utf-8',
:via => :smtp,
:via_options => {
:address => 'smtp.site.com',
:port => '25',
:user_name => 'no_reply@site.com',
:password => 'somepassword',
:authentification => :login, # :plain, :login, :cram_md5, no auth by default
:domain => "site.com" # the HELO domain provided by the client to the server
}
)
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