Campaign email  is not delivering to all subscribers.
Sent a campaign to 400+ plus subscribers but mail is delivered only to 99 subscribers.
 

The SMTP server is not delivering the emails to all subscribers.

Sending emails from an Email campaign is not solely done by Sitefinity. There are two separate parties that act in sending and delivering an email to its subscribers.

Party 1: Sitefinity sends the email and its subscribers to the SMTP server (the STMP server is user-defined in Administration -> Settings -> Advanced -> Notifications -> Profiles)
Party 2: The SMTP server is responsible to deliver the emails to subscribers

Sitefinity report of email campaign indicates this in the "Campaign Overview Report"


If the email is sent which is indicated in the "SENT TO" column. This indicates Sitefinity sent the email to the STMP server.
In column "DELIVERED" is the indication if the SMTP server sent the email to the subscriber.

If the "SENT TO" column properly indicates a number matching the number of subscribers in the mailing list used for the email campaign then Sitefinity did sent the email to the SMTP server for all subscribers.
 

If the "DELIVERED" column has a number less than the number in "SENT TO" column then this is an indication of a problem with the SMTP server.

An additional check if Sitefinity did sent all emails to the SMTP server can be done via a database query:
Query:

select * from sf_notif_msg_job

Table [sf_notif_msg_job] table holds info on all jobs (send email campaign action) for email campaigns which failed to be delivered to SMTP!!! Each send of email campaign creates one entry in this table. Each job from this table will be a success when it pushes all emails to the SMTP server with success.
Success of the job will remove it from table: [sf_notif_msg_job] and move it to table: [sf_notif_cmplt_msg_job] (completed message jobs) Successfully sent campaigns will be listed in this table.

Back to table [sf_notif_msg_job] If there is a problem completing the job (pushing the email message and subscribers) to SMTP server.
If the job is the value of column [job_status] is 4 then it means failure in sending emails or subscribers to SMTP server. Then an error will be populated in columns [failure_description].