email_server: Open port 25 and document EMAIL_GATEWAY_PATTERN.

[alexmv]

Requires zulip/zulip#35081

Fixes:

How did you test this PR?

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[timabbott]

Do we need similar changes in the kubernetes/helm files?

[timabbott]

Looks good, I wonder if we there's a good place to also document this in the README.md, with an anchor so it can be linked from the main documentation.

[timabbott]

Do we need to wait for 11.0 to be released with #35081 (and more importantly, the introduction of the new system)?