I would start using some simple diagnostics. Click the online / offline icon (two blue computer monitors in the bottom left of the Thunderbird window.)

Then try your merge. If all the mail gets to the outbox it is not the meail merge that is causing the issue.

My guess is it will be a limitation on the number of emails you can send per hour or per day set by ghandi. but it could just as possibly be your anti virus being overwhelmed bu volume and giving up the ghost causing issue. Try disabling any so called email protection t offers.

Your colleague is right.

Most email service providers (including ISPs, but most notably Yahoo, GMail, etc.) will detect multiple emails arriving at once from the same source and tag it as SPAM, then usually blacklisting your email servers.

You can inquire with Virgin in your country what their SMTP sending limit is, a quick web search returned this:

• https://help.virginmedia.com/system/templates/selfservice/vm/help/customer/locale/en-GB/portal/200300000001000/article/HELP-2180/Sending-Limits-in-Virgin-Media-Mail

ps. If you're sending email using your Gandi.net address, you should be using their SMTP server.

Their FAQ is very clear about their limitations:

• Max number of 'to' addresses when sending from email client (Apple Mail, Thunderbird, Outlook etc.): 40

You could contact their support to see if this started being enforced recently and if it applies to your account.