You need to make sure that your ruby/openssl version supports TLS version greater than 1.0 which should be the case unless you're using a very old version of ruby or openssl - support for TLS 1.1 & 1.2 was added back in 2012: ruby/ruby@060184c .

The best way to check if you're safe is to connect to the new temporary endpoint and send an email:

require 'postmark'

client = Postmark::ApiClient.new(SERVER_TOKEN, host: 'api-ssl-temp.postmarkapp.com')
client.deliver(from: ..., to: ...., text_body: 'test')

In case of any issues with ruby and SSL I recommend this troubleshooting guide from the bundler project: https://bundler.io/guides/rubygems_tls_ssl_troubleshooting_guide.html#tls-10-and-11-are-deprecated .

EDIT:
⚠️ Also, make sure you're using the latest version of this gem: 1.21.3. ⚠️

Thanks @tomazy 👍

You may need to add http_ssl_version: :TLSv1_2 to ApiClient:

client = Postmark::ApiClient.new(POSTMARK_API_TOKEN, host: 'api-ssl-temp.postmarkapp.com', http_ssl_version: :TLSv1_2)

In case you use postmark-rails, add http_ssl_version: :TLSv1_2 to your application config file:

config.action_mailer.postmark_settings = {
  api_token: ENV['POSTMARK_API_TOKEN'],
  http_ssl_version: :TLSv1_2,
}

You may need to add http_ssl_version: :TLSv1_2 to ApiClient:

client = Postmark::ApiClient.new(POSTMARK_API_TOKEN, host: 'api-ssl-temp.postmarkapp.com', http_ssl_version: :TLSv1_2)

In case you use postmark-rails, add http_ssl_version: :TLSv1_2 to your application config file:

config.action_mailer.postmark_settings = {
  api_token: ENV['POSTMARK_API_TOKEN'],
  http_ssl_version: :TLSv1_2,
}

Please note that http_ssl_version is needed only in postmark gem versions prior to v1.21.3. Version 1.21.3 removes TLSV1 as the default SSL version and lets the HTTP client negotiate the SSL version.

Method: HTTPClient::SSLConfig#ssl_version

See: 529e9db

I may be mistaken here, but I ran into an exception when following the testing steps above. Despite the bundler curl test passing 👌. After updating our postmark(-rails) gem test email sent fine.

Just in case someone runs into this exception and thinks they need to fix other things - check that your postmark gem is up to date.

Before gem update:

client = Postmark::ApiClient.new k, host: 'api-ssl-temp.postmarkapp.com'
=> #<Postmark::ApiClient:0x000055daf3263260 ...>>>
client.deliver(from: '...', to: '...', text_body: 'testing api-ssl-temp.postmarkapp.com')
Traceback (most recent call last):
        1: from (irb):14
OpenSSL::SSL::SSLError (SSL_connect SYSCALL returned=5 errno=0 state=SSLv3/TLS write client hello) 

After update:

client.deliver(from: '...', to: '...', text_body: 'testing api-ssl-temp.postmarkapp.com')
=> {:to=>"...", :submitted_at=>"2021-02-25T13:50:46.4864111Z", :message_id=>"...", :error_code=>0, :message=>"OK"}

after upgrading to the latest postmark gems, I had to install openssl gem, install libssl-dev package and recompile the ruby binary - all good now