Senior ML Engineer - Email Security (remote work)

CloudLinux Inc.; Comunidad de Madrid; Remote; EUR 70,000 - 120,000; Full time

Benefits offered by this job

Fully remote work with flexible hours

Paid 24 days vacation per year

10 national holidays

Unlimited sick leaves

Private medical insurance

Co-working and gym reimbursement

Education budget

Patent-worthy idea rewards

Job summary

CloudLinux Inc. is seeking a Senior Machine Learning Engineer to join the remote-first team. You will drive development, evaluation, and production rollout of Imunify Email security solutions, focusing on spam-detection models, Go-based engine components, and corpus tooling.

The role emphasizes production ML, transformer training in PyTorch, and strong communication skills in English, with remote work globally and a focus on scalable security infrastructure.

Qualifications

• 5+ years shipping detection or classification systems with false-positive cost in mind.
• At least one ML classifier taken to production: corpus construction, threshold selection, error analysis.
• Hands-on transformer training in PyTorch: tokenizers, pre-training, fine-tuning, CPU inference.
• Linux, shell, and Python on shared servers and a shared GPU.
• English at upper-Intermediate (B2) with strong written communication for docs and code reviews.

Responsibilities

• Train, evaluate, and release spam-detection models: build the corpus, pick the threshold, read the precision/recall report, decide ship or retrain.
• Build the production side of the new engine in Go: the inference service, the mail-filter integration, the tokenizer, the streaming message parser.
• Fix detection quality end to end, from customer feedback to rule/corpus fix or retrain.
• Keep the corpus tooling and training infrastructure running.
• Review and approve classifier, rule, and model changes for releases.
• Answer partner engineers who ask why a message scored the way it did.
• Write specs, handbooks, and post-mortems.

Skills

Spam detection experience

Production ML model

Transformer training PyTorch

Linux and Python

English (B2) written

AI tooling

Tools

Go/Rust for systems

rspamd internals

Job description

CloudLinux is a global remote-first company. We are driven by our principles: do the right thing, employees first, we are remote first, and we deliver high-volume, low-cost Linux infrastructure and security products that help companies to increase the efficiency of their operations. Every person on our team supports each other and does what we can to ensure we all are successful.

Check out our website for more information: https://cloudlinux.com/

CloudLinux Inc. develops the Imunify family of security products, designed to protect hosting environments from a wide range of cyber threats. One of these solutions is Imunify Email, an email security product that protects hosting servers against spam, phishing, and other email-based threats.

Imunify Email filters mail on about 5,000 hosting servers. Today the filter is rspamd with rules and a scoring model we train in-house. We are replacing it with a transformer we trained ourselves that runs on the customer's own CPU in under 100 MB and already beats rspamd at every operating point we measured. We are looking for a Senior Machine Learning Engineer to join our team and work alongside the engineer currently responsible for both the existing spam-filtering pipeline and the new ML model. Together, you will drive the development, improvement, and production rollout of our email security technology.

What you will do:

• Train, evaluate, and release spam-detection models: build the corpus, pick the threshold, read the precision/recall report, decide ship or retrain.
• Build the production side of the new engine in Go: the inference service, the mail-filter integration, the tokenizer, the streaming message parser.
• Fix detection quality end to end, from a customer's "this phishing got through" to a rule, a corpus fix, or a retrain, with the false-positive cost weighed against the catch.
• Keep the corpus tooling and training infrastructure running.
• Review and approve classifier, rule, and model changes, so no release waits on one person.
• Answer partner engineers who ask why a message scored the way it did.
• Write specs, handbooks, and post-mortems. That is how this team transfers knowledge.

Your first six months:

• Month 1: you ship a rule-model release on your own.
• Month 3: your first piece of the new engine is merged and running on staging.
• Month 6: a model release goes out while the principal engineer is away.

What you bring:

• 5+ years of shipping detection or classification systems where false positives have a cost you had to defend: anti-spam, anti-phishing, anti-malware, fraud, abuse.
• You have taken at least one ML classifier to production yourself: corpus construction, threshold selection, error analysis.
• Hands-on transformer training in PyTorch: tokenizers, masked-language pre-training, fine-tuning, ONNX export, CPU inference measurement.
• Linux, shell, and Python on shared servers and a shared GPU.
• Daily usage of AI coding agents and tools.
• English at upper-Intermediate (B2) level or higher, with particularly strong written communication skills for technical documentation, specifications, and code reviews. Your specs and merge-request descriptions count as much as your code.

It is a strong plus if you have:

• Email internals: SMTP, MIME, transfer encodings, SPF / DKIM / DMARC, how Exim, Postfix, and Dovecot handle mail to a filter.
• Experience in creating embedded models
• Go or Rust for systems code: parsers, streaming, memory budgets, concurrency.
• rspamd or SpamAssassin internals: rules, composites, Lua plugins, mass-check scoring.
• Multi-million-sample corpora, labelling pipelines, annotator tooling.
• Small models for constrained hardware: quantization, distillation, latency budgets.
• LLM-based labelers or validators for scaling annotation.
• Hosting panels: cPanel, Plesk, DirectAdmin.

What's in it for you?

• A focus on professional development.
• Interesting and challenging projects.
• Fully remote work with flexible working hours, that allows you to schedule your day and work from any location worldwide.
• Paid 24 days of vacation per year, 10 days of national holidays, and unlimited sick leaves.
• Compensation for private medical insurance.
• Co-working and gym/sports reimbursement.
• Budget for education.
• The opportunity to receive a reward for the most innovative idea that the company can patent.

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