Why does my email look fine in Gmail but broken in Outlook, and how do I test this properly before sending?

Question

I am sending transactional emails from my app and just found out a signup confirmation email that looked perfect in Gmail was completely broken in Outlook, images misplaced and layout collapsed. I do not have easy access to test across every email client before sending.

Is there a good way to understand why email rendering is so inconsistent across clients, and what tools exist to test this properly before I embarrass myself in front of more users?

Bobby · Answer

Hi there, Welcome to the classic pain of HTML email. The core reason for the inconsistency: most email clients use ancient or partial rendering engines instead of a modern browser engine. Outlook desktop specifically renders HTML using Microsoft Word’s engine, not a real browser, which explains why modern CSS (flexbox, grid, even basic margin/padding in some cases) behaves unpredictably. Practical rules that avoid most of the pain: Use table-based layouts instead of divs with flexbox or grid. It looks outdated but it is genuinely the most reliable cross-client approach. Inline all your CSS rather than using a

alexdo · Answer

Heya, the rendering advice is right, Outlook desktop using the Word engine really is the root of most of it. Worth being more specific on the SMTP point at the end though, because the details are where people get caught. DigitalOcean blocks three ports, not just one: 25, 465 and 587. The 587 part surprises people, since it is the modern submission port and the one most provider setup guides hand you first. DigitalOcean’s support documentation also notes the block “applies to all Droplets by default and includes traffic passing through a Reserved IP address,” so attaching a Reserved IP does not route around it, which is a common assumption worth killing early. Given that, the cleanest path off a Droplet is a provider’s HTTPS API rather than SMTP at all, since port 443 is never part of this problem. SMTPfast is what I use for one of my own domains and it fits this case well: it is built on Amazon SES, so you inherit SES deliverability without dealing with SES directly, and it exposes a JSON REST API alongside an SMTP bridge for legacy apps that cannot be changed. If you do go the SMTP bridge route rather than the API, check which port it listens on against that blocked list first, I could not find the port documented publicly, and that is the one detail that decides whether it works from a Droplet without further thought. It is also Resend-compatible on the sending API, so if you already wrote against Resend, it is close to a base URL and key swap rather than a rewrite. Regards

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Why does my email look fine in Gmail but broken in Outlook, and how do I test this properly before sending?

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Why does my email look fine in Gmail but broken in Outlook, and how do I test this properly before sending?

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