Thunderbird not sending emails with moderate-sized attachments, no error message is given.
I'm having an unusual problem that I've tracked down to Thunderbird. When I send an email with an attachment which is small,(~1 Mb) it gets through to the user no problem. When I send an attachment which is larger (~10 Mb) but still below the threshold of both the outgoing smtp server and the recipients incoming server, (25 Mb), the message sends without error, but it never gets to the recipient. There is no error message on the other end either, and no email is sent citing things as undeliverable. It just disappears.
As a test if I send an message from Thunderbird, which is configured (see below) from the account I have with my ISP to my gmail account (web interface), with a 14.5 Mb attachment, the email seems to be sent, but it never shows up at gmail (nor in spam). If I send the same email using the web interface for the address I have with my ISP to my gmail (again, using the web interface for the gmail), it shows up instantly. I have confirmed with my ISP that all the smtp settings are correct. Repeating the above scenarios with small attachments or no attachments works instantly in all cases.
This appears to be an issue with Thunderbird sending the attachment, and its creating a real problem because I keep having to follow up with people to see if the attachments get through as I have not figured out the threshold message size where the behavior changes between allowing the message and just ghosting it.
I am using Thunderbird 52.6 but also saw the behavior in 52.1.1. The submission port is 25, which is unblocked by my ISP and is the port I'm supposed to be using. The authentication method is Normal Password and the connection security is STARTTLS. The ISP has verified this is correct.
I have Symantec Endpoint protection installed on my machine, and it is not clear to me if it is scanning outgoing attachments, but the behavior is identical if Symantec is on or completely disabled.
I am at a loss. Has anybody else seen this behavior? Is there a fix?
Chosen solution
you have checked your SMTP and the received for the size limits, but there were probably a number of intermediate steps before the mail went to where it was going. Each of those will have a size limit as well.
Then there is the size of the mail Vs the size of the attachment. and for added confusion different ways of measuring size.
When you create a mail on a web interface, they invariably use the "file size" to determine if it is small enough, even though the file will be mime encoded before dispatch. When you use an SMTP server they use the mime encoded size of the files. Usually about 33% larger, so your 14.5Mb attachment will be roughly 19Mb when encoded for sending. Then there is the size of the actual massage itself, if you are copying from say word then it is not unusual for the size of that to exceed a Mb in size so suddenly your message is more than 20mb in size.
It is also possible one of the servers along the way drops you mail as it is submitted on port 25, but I would not expect mail size to affect that sort of spam testing.
I would suggest you try logging the sending of the mail. https://wiki.mozilla.org/MailNews:Logging#Windows
IF you need help reading it you can email it to me. DO NOT upload to the forum. IF the log shows the mail going without any sort of error, you will need to talk to your provider and have them look at their SMTP server logs as to what happened after Thunderbird handed the mail off to their server.
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you have checked your SMTP and the received for the size limits, but there were probably a number of intermediate steps before the mail went to where it was going. Each of those will have a size limit as well.
Then there is the size of the mail Vs the size of the attachment. and for added confusion different ways of measuring size.
When you create a mail on a web interface, they invariably use the "file size" to determine if it is small enough, even though the file will be mime encoded before dispatch. When you use an SMTP server they use the mime encoded size of the files. Usually about 33% larger, so your 14.5Mb attachment will be roughly 19Mb when encoded for sending. Then there is the size of the actual massage itself, if you are copying from say word then it is not unusual for the size of that to exceed a Mb in size so suddenly your message is more than 20mb in size.
It is also possible one of the servers along the way drops you mail as it is submitted on port 25, but I would not expect mail size to affect that sort of spam testing.
I would suggest you try logging the sending of the mail. https://wiki.mozilla.org/MailNews:Logging#Windows
IF you need help reading it you can email it to me. DO NOT upload to the forum. IF the log shows the mail going without any sort of error, you will need to talk to your provider and have them look at their SMTP server logs as to what happened after Thunderbird handed the mail off to their server.
Checking the log file confirmed that the message is getting sent successfully to the smtp server at the ISP. I'm taking this information back to them. Thanks so much for pointing me toward logging!