Email return receipt - Githubissues

Hi, I have the following error when I click the "Email return receipt" button

I, [2019-06-19T11:43:28.473575 #4856]  INFO -- :   Rendered layouts/_footer.html.erb (0.7ms)
I, [2019-06-19T11:43:28.473787 #4856]  INFO -- : Completed 200 OK in 29ms (Views: 17.2ms | ActiveRecord: 2.2ms)
I, [2019-06-19T11:43:33.664686 #4856]  INFO -- : Started GET "/reservations/4/send_receipt" for 10.2.5.78 at 2019-06-19 11:43:33 +0200
I, [2019-06-19T11:43:33.666155 #4856]  INFO -- : Processing by ReservationsController#send_receipt as HTML
I, [2019-06-19T11:43:33.666217 #4856]  INFO -- :   Parameters: {"authenticity_token"=>"0pRVw4cWRtWxX3bsbXFWfHEwDSD28l9Pc9IfBMvgm7WW8r4GCc6F1mtYAnhmjvaR31+HDMqhxS1bJ047YE2nrA==", "id"=>"4"}
I, [2019-06-19T11:43:33.679536 #4856]  INFO -- :   Rendering user_mailer/reservation_status_update.html.erb
I, [2019-06-19T11:43:33.680091 #4856]  INFO -- :   Rendered user_mailer/_returned.html.erb (0.0ms)
I, [2019-06-19T11:43:33.681979 #4856]  INFO -- :   Rendered user_mailer/reservation_status_update.html.erb (2.4ms)
I, [2019-06-19T11:43:33.684599 #4856]  INFO -- : Sent mail to XXXX@YYYY.ZZ (2.1ms)
I, [2019-06-19T11:43:33.684806 #4856]  INFO -- : Completed 500 Internal Server Error in 18ms (ActiveRecord: 1.6ms)
F, [2019-06-19T11:43:33.685558 #4856] FATAL -- :
F, [2019-06-19T11:43:33.685602 #4856] FATAL -- : Errno::ECONNREFUSED (Connection refused - connect(2) for nil port 25):
F, [2019-06-19T11:43:33.685651 #4856] FATAL -- :
F, [2019-06-19T11:43:33.685681 #4856] FATAL -- : app/controllers/reservations_controller.rb:433:in `send_receipt'

I checked and Reservation server does not try to connect to the smtp server.

YaleSTC / reservations

Email return receipt #1773