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How do I make external emails stand out?

MaxMail adds a hidden tag (X-External-Sender) to every email that comes from outside your company. Desktop mail apps can turn that tag into a colour highlight.

Thunderbird

1. Settings → General → Tags → New… → name it External Mail, pick orange or red → OK
2. Tools → Message Filters → New…
3. Click the first dropdown (defaults to "Subject"), scroll to the bottom → Customize… → type X-External-Sender → Add → OK
4. Match: X-External-Sender | contains | yes
5. Action: Tag Message → External Mail → OK

Tip: Thunderbird runs filters in order. If you already have many filters and the tag doesn't appear, make sure the filter rule is at the TOP of the list — an earlier filter can block it.

Outlook (classic desktop app)

Outlook can't match a hidden header in Conditional Formatting, so use a rule that tags external mail with a coloured category:

1. Home → Rules → Manage Rules & Alerts → New Rule…
2. Under Start from a blank rule, pick Apply rule on messages I receive → Next
3. Tick with specific words in the message header → click the underlined specific words link → type X-External-Sender → Add → OK → Next
4. Tick assign it to the category category → click the underlined category link → New… → name it External Mail, pick orange or red → OK → OK
5. Next (skip exceptions) → tick Run this rule now on messages already in Inbox (optional) → Finish

The rule colours external mail automatically from now on.

iPhone / Android / Apple Mail

Mobile mail apps don't support colour-coding by a custom header, so the tag isn't visible there. The tag still sits in every external message, and all the protection (spam & virus filtering, sender checks) works on every device regardless.

MaxMail Webmail (SOGo)

Doesn't colour-code external mail natively — a display limitation, not a protection gap.