What about attachments size?

[testphoneur-tech]

Hello!

Going through the Graph API, there is a 3Mb limit on attachements unless chunking is implemented. Does this proxy do chunking? Or is it using the Exchange API (which has a 35Mb limit)?

Thanks

[simonrob]

The proxy doesn't use any API in this sense. Instead, it intercepts IMAP/POP/SMTP requests and adds the correct OAuth header.

[testphoneur-tech]

Hi Simon, thanks for the reply. My questions might need to be more precise.

I am following Microsoft's guidance to use App Registrations to send SMTP emails :
https://learn.microsoft.com/en-us/exchange/client-developer/legacy-protocols/how-to-authenticate-an-imap-pop-smtp-application-by-using-oauth#add-the-pop-imap-or-smtp-permissions-to-your-microsoft-entra-application

It says to grant the App Registration SMTP.SendAsApp permission, and do some PowerShell commands to create the service principal and delegated permissions in Exchange.

My understanding is that using this SMTP.SendAsApp permission opens the Exchange API for SMTP, allowing to get an OAuth token and then send through normal SMTP protocol (smtp.office365.com) using that token as credential. This has a limit of 35Mb attachments.

Is this the setup email-oauth2-proxy works with?

PS.: There is another way to send emails through the Graph API > Application permission > Mail.send, which is more complicated because it has a limit of 3Mb attachments unless the application that sends the emails through it, uses chunking. Once this chunking logic is implemented the limit is 150Mb attachments. Does email-oauth2-proxy do chunking?

Thanks!

[simonrob]

Microsoft's documentation is unclear in many areas, but ultimately, all of the bits around app registrations and SMTP AUTH etc just refer to enabling SMTP as a protocol, and allowing an OAuth client (ie the proxy) to use it.

The proxy does not interfere with the connection in any way beyond the authorisation step, so attachment size is limited to whatever the provider's SMTP limit is. As you note, there are other ways of submitting emails, such as the graph API, but these are a totally different approach.