Hi Robin,
Let me list the facts for this case
1. sent column is cleared
2. hourly trigger writes a new entry in send status
3. there is no copy of this mail in the gmail send folder
if you manually run the process after 1. it all works fine.
If 2. happened theb the mail was to be accepted by the mailrelay. If 3. happened it has somehow been rejected.
Are you sure the google/gmail account which configured FormMule is the same as the sheet-owner running the triggers?
If you logon with your account and manually have no issues your grants may differ then of the sheet.
The issue might even be beyond your control if you have a business/education account being managed by others which can set policies to prohibit automated mailprocesses.
Disregard the previous if you are solely working with a personal Google account and are sheetowner and did the Formmule config with the same.
Final thought for now: i create columns to verify my data is valid and complete.
It reports 'INCOMPLETE' when stuff fails - like the emailaddress or so -
'LAUNCH' when complete and sent status is empty,
and 'DONE' when sent status contains a logentry.
FormMule has for each template it's own LAUNCH trigger configured.
And to prevent running into the daily mail quotum I report DELAYED when i submitted more than 100 mails in the last 24 hours.
If you have more FM configurations running this is not enough but another problem ;)
I hope my insights might help you finding the culprit.
Cheers, Bart