Your software
My Mautic version is:
2.16.0
My PHP version is:
7.2.27-6
My problem is:
Since the update to 2.16.0 Mautic is attaching the tracking pixel in emails +1 for every recipient.
Every next email gets the attachments from the one before.
So the first recipient has i.e 2 images in the email plus the tracking pixel, the second one has then 4 images plus two tracking pixels, the third gets 6 images plus 3 tracking pixels and so on.
Usually the system stops sending emails when there are like 300 attachments, because then the email is too big.
That didn’t happen before. And I don’t really understand why it happens now.
And it happens by sending as broadcast as well as sending in the foreground.
It happens with template and segment emails and it happens also when sending a campaign.
These errors are showing in the log:
mautic.ERROR: [MAIL ERROR] Expected response code 250 but got code “552”, with message “552 5.3.4 Message is too long. " Log data: >> MAIL FROM:alex@hartmut.io << 250 Ok >> RCPT TO:hammerschmied.alex+test3@gmail.com << 250 Ok >> DATA << 354 End data with . >> . << 552 5.3.4 Message is too long. !! Expected response code 250 but got code “552”, with message “552 5.3.4 Message is too long. " (code: 552) (send); hammerschmied.alex+test3@gmail.com {“exception”:”[object] (Swift_TransportException(code: 552): Expected response code 250 but got code “552”, with message “552 5.3.4 Message is too long.\r\n”\nLog data:\n>> MAIL FROM:alex@hartmut.io\r\n\n<< 250 Ok\r\n\n>> RCPT TO:hammerschmied.alex+test3@gmail.com\r\n\n<< 250 Ok\r\n\n>> DATA\r\n\n<< 354 End data with .\r\n\n>> \r\n.\r\n\n<< 552 5.3.4 Message is too long.\r\n\n!! Expected response code 250 but got code “552”, with message “552 5.3.4 Message is too long.\r\n” (code: 552) at /var/www/html/mautic/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Plugins/LoggerPlugin.php:140)”}
Steps I have tried to fix the problem:
I turned of the tracking pixel, then the images are also duplicated as attachments 
