A partial send.
THE COMMON CAUSES
A plan limit reached partway The segment smaller than you thought Bounces, so some addresses failed Some messages filtered to spam, so recipients did not see them The platform throttling delivery, so it is still in progress
CHECK THE REPORT
Delivered versus sent tells you how many actually reached a server.
A large gap indicates bounces or rejections.
IF IT IS STILL SENDING
Large campaigns are delivered over time rather than instantly. Wait before concluding anything.
IF DELIVERY IS LOW
Check bounce rates. A high hard bounce rate means your list contains invalid addresses.
Check whether a particular provider rejected everything, which points at a reputation or authentication problem with that provider.
THE PROVIDER PATTERN
If every address at one provider failed while others succeeded, the problem is your standing with that provider.
Check authentication and blacklists.
WHAT NOT TO DO
Resend immediately to everyone. Those who received it get it twice, which produces complaints.
Resend only to those who did not receive it, if the platform allows.
