Check the schedule and queue
Confirm which time zone the schedule uses and whether it follows daylight-saving changes. Compare queued, processing, and accepted timestamps to distinguish application delay from delivery latency after handoff.
Large campaigns may be released in batches or throttled to protect sending reputation or comply with account limits. A provider may retry temporary recipient-server errors, spreading delivery over time.
Diagnose delay at the recipient
Compare delivery events across recipients and domains. If the sending service accepted a message promptly but one destination received it late, inspect available events and the recipient's spam or quarantine controls.
For time-critical messages, avoid promising second-level delivery and use an independently verified channel when appropriate. Send a small test ahead of a campaign and check schedule, time zone, and queue estimates.