Exchange Online IMAP attachment download delay and failure only for a specific account

Question

We are using a JavaMail-based application to retrieve emails from Exchange Online via IMAP. Since June 30, we have been facing an issue, but only for a specific account:

Attachments download normally when the mailbox has few messages

As the number of emails grows, attachment download becomes slower

Eventually, the application fails to retrieve the emails

The same application works fine with other accounts (no delay)

Example log:

[INFO ][2025/06/30 15:43:50.726] AttachSize : 10602462
[INFO ][2025/06/30 15:43:55.822] buffer size : 1048576
[INFO ][2025/06/30 15:43:58.292] buffer size : 2097152
[INFO ][2025/06/30 15:44:02.324] buffer size : 3145728
[INFO ][2025/06/30 15:44:04.603] buffer size : 4194304
[INFO ][2025/06/30 15:44:07.340] buffer size : 5242880
[INFO ][2025/06/30 15:44:13.099] buffer size : 6291456
[INFO ][2025/06/30 15:44:16.010] buffer size : 7340032
[INFO ][2025/06/30 15:44:18.509] buffer size : 7747951 [complete]

Attachments of the same size normally take about 3 seconds to download, but after the delay started, it takes about 30 seconds.

Outlook/OWA works fine, but IMAP (JavaMail) shows this delay and failure only for the specific account.

Could such account-specific behavior happen in Exchange Online IMAP? Any advice on what to check (server-side or client-side) would be appreciated.

Answer 1

Hi @ktoa 
Thank you for reaching out to Microsoft Learn Q&A!  
Based on your description, I understand that your JavaMail application experiences slower performance when retrieving emails via IMAP from Exchange Online, especially as the mailbox size increases. 

From my research, one possible explanation for this behavior is the throttling policy applied in Exchange Online. This policy is enforced per mailbox and is designed to manage resource consumption, prevent system overload, and ensure stable performance for all users and applications. 

Server-side considerations: 
To check the server side: In order to examine the throttling policy applied to IMAP in Exchange Online, you’ll need to contact Microsoft Support by opening a support ticket via the Microsoft Admin Center or by calling the Support Number. This is in line with the guidance provided in the referenced thread: is any option to change imap throttling policy? - Microsoft Q&A 

Client-side suggestions: 
To improve performance on the JavaMail side, you might consider adding the following properties to your application: 

props.setProperty("mail.imaps.partialfetch","false");
props.setProperty("mail.imaps.fetchsize", "1048576"); // or higher

These settings can help optimize how messages are fetched, especially for larger mailboxes. 

Reference: java - Setting the "mail.imaps.fetchsize" property to a large number using JavaMail have any impact… 

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If the issue persists after trying the above methods, could you kindly share any screenshots of the error codes or messages you're encountering? 

It would help me better understand your situation. 

Please know that while our initial response might not resolve the issue right away, your input is incredibly valuable. With a bit more detail, we’ll work together to find the best solution for you. 

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