It shows as sent on their end but they haven't read it yet.
Tips
grammar
已 (yǐ) is a literary/written 'already' marking completed action — common in formal status labels. The everyday spoken equivalent is 已经 (yǐjīng). Compare 已读 (yǐdú, 'read') — the 'seen' indicator on WeChat-like apps.
usage
Standard message-status label across most apps and email clients. Sequence is typically: 发送中 (sending) → 已发送 (sent) → 已送达 (yǐsòngdá, delivered) → 已读 (read).
