IMAP mailbox names that contain non-ASCII characters use modified UTF-7 (RFC 3501). Inside those names, a Base64-like alphabet encodes Unicode with two important differences from RFC 4648:
| Value | Standard Base64 | IMAP variant |
|---|---|---|
| 62 | + |
+ |
| 63 | / |
, |
| Padding | = required |
none |
The / character is replaced with , because / is the IMAP hierarchy separator. Padding is omitted so mailbox names stay compact.
Example
ASCII mailbox names need no encoding. A name with non-ASCII characters embeds &…- segments where … is IMAP Base64 of UTF-16BE bytes.
When developers need this
- Debugging mailbox names returned by IMAP servers in Europe (umlauts, accents, Cyrillic).
- Migrating mailboxes between providers.
- Writing IMAP clients that round-trip international folder names.
Use the standard detector to recognise IMAP Base64, then normalize to standard Base64 or decode with the IMAP standard selected.
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