 <p>Finally am able to find the actual cause of above issue.</p> <p>I am using the gem <a href="https://github.com/wildbit/postmark-rails" rel="nofollow">postmark-rails</a>, previously postmark was not supporting the email-attachments, so recently they had enhanced gem to support attachments and unfortunately i was using the old version of it, so i need to update gem version to latest as they have mentioned in one of their issues: <a href="https://github.com/wildbit/postmark-rails/issues/25" rel="nofollow">attachment-issue</a></p> <p>also i was trying to send url of file saved on S3, so instead of that i need to read that file from url and then send it as attachment</p> <pre><code> require "open-uri" def send_refer_pt_request(params, attachment) if attachment.present? mime_type = MIME::Types.type_for(attachment).first url_data = open(attachment).read() attachments["#{attachment.split('/').last}"] = { mime_type: mime_type, content: url_data } end mail( to: &lt;some_email&gt;, subject: "Refer Request", tag: "refer_request") end </code></pre>