Best Practices

Message Construction

Always include both a text/plain and a text/calendar MIME part. The text/plain part serves as a fallback for email clients that do not process iMIP. Wrap them in multipart/alternative so the client can choose the best representation.

Set the Content-Type header on the calendar part to text/calendar; method=REQUEST; charset=UTF-8 (or whichever iTIP method applies). The method parameter must match the METHOD property inside the iCalendar data.

Include a valid UID and increment the SEQUENCE number for every update to an existing event. Clients use UID to correlate related messages and SEQUENCE to determine which version is current.

Sending

Use the organizer’s email address as both the SMTP MAIL FROM and the ORGANIZER property value. The ATTENDEE property must contain the recipient’s email address so the receiving client can match the invitation to the correct calendar.

Send iTIP replies (REPLY method) to the organizer only, not to all attendees. The organizer is responsible for aggregating responses and sending updated REQUESTs if necessary.

Processing Incoming Messages

When receiving an iMIP message:

1. Extract the text/calendar MIME part.
2. Validate that the METHOD parameter matches the METHOD property inside the iCalendar object.
3. Match the ATTENDEE value against the user’s known email addresses.
4. Check the UID and SEQUENCE against existing calendar entries to determine if this is a new event, an update, or a stale message.
5. Apply the scheduling operation according to iTIP rules.

Common Pitfalls

• Missing or mismatched METHOD — Without the method parameter in Content-Type, many clients will not process the calendar part.
• Broken encoding — Always use UTF-8 for the calendar part. Non-ASCII characters in SUMMARY or LOCATION must be properly encoded.
• Duplicate UIDs — Never reuse a UID across different events. Use a UUID or similar unique identifier.
• Ignoring SEQUENCE — Sending an update without incrementing SEQUENCE causes clients to silently ignore the new version.