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sending meeting request as Ical files using new SMTP auth

Adequasys 0 Reputation points
2025-06-10T08:27:42.0433333+00:00

Since using SMTP mail sending with Basic auth will be phased-out, we are working to transition with an alternative. We are  trying both Graph API and SMTP with oauth, and we are facing some issues with a critical use case of our app, that is sending meeting request using an attached IcalFile.

We discover that with the 2 solutions, we can no more receive an ica file properly displayed as a meeting request in outlook (even it works fine in other email client like gmail)

Since the problemn seem lionked to the way email is processed and sent, can we have any help and solutions on this issue ? Please fin below some details about the tryout we did, and the behavior we observed.

We can adapt some option on how we use emails, how be build ICS, but we need to keep sending ICS files, since we need to be able to send email using other providers, using regular SMTP servers.

Regards.

CASE 1 Configuration : SMTP with oauth / - ".ics" file send as "attachment" Results

  • Outlook : attachment visible, but not interpreted as meeting request ICS part visible in raw format when downloading message
  • Gmail : all is fine

(see attached file "SMTP & STARTTLS - attachment.pdf")

CASE 2

Configuration : SMTP with oauth / - ".ics" file send as "inline"

Results

  • Outlook : no attachment nor meeting request visible

ICS part not visible when doanwsloding message, it seems have been supressed

  • Gmail : no attachment nor meeting request visible

ICS part not visible when doanwsloding message, it seems have been supressed

(see attached file "SMTP & STARTTLS - inline.pdf")

CASE 3

Configuration : sending mail with GraphAPI / - ".ics" file send as "attachment"

Results

  • Outlook : attachment visible, but not interpreted as meeting request
  • Gmail : all is fine

(see attached file "GRAPH API - attachment")

CASE 4

Configuration : sending mail with GraphAPI / - ".ics" file send as "attachment"

Results

  • Outlook : no attachment nor meeting request visible ICS file part visible when downloading raw message
  • Gmail : all is fine

(see attached file "GRAPH API - inline")

Since using SMTP mail sending with Basic auth will be phased-out, we are working to transition with an alternative.
We are  trying both Graph API and SMTP with oauth, and we are facing some issues with a critical use case of our app, that is sending meeting request using an attached IcalFile.

We discover that with the 2 solutions, we can no more receive an ica file properly displayed as a meeting request in outlook (even it works fine in other email client like gmail)

Since the problemn seem lionked to the way email is processed and sent, can we have any help and solutions on this issue ?
Please fin below some details about the tryout we did, and the behavior we observed.

We can adapt some option on how we use emails, how be build ICS, but we need to keep sending ICS files, since we need to be able to send email using other providers, using regular SMTP servers.

Regards.

CASE 1
Configuration : SMTP with oauth / - ".ics" file send as "attachment"
Results

  • Outlook : attachment visible, but not interpreted as meeting request
    ICS part visible in raw format when downloading message
  • Gmail : all is fine

(see attached file "SMTP & STARTTLS - attachment.pdf")

CASE 2

Configuration : SMTP with oauth / - ".ics" file send as "inline"

Results

  • Outlook : no attachment nor meeting request visible

ICS part not visible when doanwsloding message, it seems have been supressed

  • Gmail : no attachment nor meeting request visible

ICS part not visible when doanwsloding message, it seems have been supressed

(see attached file "SMTP & STARTTLS - inline.pdf")

CASE 3

Configuration : sending mail with GraphAPI / - ".ics" file send as "attachment"

Results

  • Outlook : attachment visible, but not interpreted as meeting request
  • Gmail : all is fine

(see attached file "GRAPH API - attachment")

CASE 4

Configuration : sending mail with GraphAPI / - ".ics" file send as "attachment"

Results

  • Outlook : no attachment nor meeting request visible
    ICS file part visible when downloading raw message
  • Gmail : all is fine

(see attached file "GRAPH API - inline")

Attachments :

https://share.adequasys.com/link/l2Y3EnJg7FAUsCOTZBQC0L

Microsoft Graph
Microsoft Graph
A Microsoft programmability model that exposes REST APIs and client libraries to access data on Microsoft 365 services.
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  1. Aashutosh Tiwari - MSFT 435 Reputation points Microsoft External Staff
    2025-06-10T10:01:51.21+00:00

    Hi Adequasys,

    Thanks for posting question on Microsoft Forum.

    One potential solution is to ensure that the Content-Type header is correctly set when sending the ICS file. Some users have had success by explicitly setting it to:

    Content-Type: text/calendar; method=REQUEST; charset=UTF-8

    This helps Outlook recognize the ICS file as a meeting request rather than a simple attachmen

    Hope this helps.

    If the answer is helpful, please click Accept Answer and kindly upvote it. If you have any further questions about this answer, please click Comment

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