Read the official Contao announcements.
Resuming interrupted mailings
by Leo Feyer – From practice
Unfortunately, the description of how to resume an interrupted newsletter mailing is wrong in both the Contao back end and the official Contao book. You have to enter the number of the next recipient instead of the number of the next sending cycle in the "start cycle" field.
Viewing the log file
Each mailing is logged in a separate file in the system/logs directory. The file is named newsletter_*.log with the wildcard being the ID of the respective newsletter. In the log file, you can track how many mails have been sent already (one recipient per line).
Resuming the mailing
The documentation incorrectly states that you have to enter the number of the next sending cycle depending on the number of mails per cycle in the start cycle field. However, you have to enter the number of the next recipient. E.g., if the sending process is interrupted after 120 mails, you have to enter "120" in the start cycle field to continue with the 121st recipient (counting starts at 0).
Adjustment of the explanation
From version 2.9.4, the explanation in the Contao back end will be adjusted accordingly and the field will be renamed from start cycle to offset.