电子邮件收件箱文件夹中UID的最大值是多少？

Question

I'm working on an application that uses the UID to fetch emails from exchange server. The UID appears to be only 6 digits long. Does that mean its maximum value is 999999? It's possible my inbox will reach over 1 million emails and I need a way to uniquely identify each email in the Inbox folder.

Answer 1

By the RFC , it can be up to a 32-bit number, so 4,294,967,295:

Unique Identifier (UID) Message Attribute: A 32-bit value assigned to each message, which when used with the unique identifier validity value (see below) forms a 64-bit value that MUST NOT refer to any other message in the mailbox or any subsequent mailbox with the same name forever.

If you've had more than 4 billion emails in the past, the server will be forced to reindex, and change the UIDVALIDITY , invalidating any client caches. If you currently have more than 4 billion, this cannot be strictly represented by an IMAP server.

Any given server implementation may have lower limits. The numbers aren't zero padded, so it's likely you just haven't had a million messages yet. When you do, you'll start seeing seven-digit UIDs.