Where those attachments uploaded completely before sending?
You can see the upload progress with the red background of the file name.

Thanks for your response, it goes up perfectly, the error is when you make the shipment, the mail arrives at the recipient, but the attachments remain in draft, I think it is a session problem, I have an https balancer, round robin mode, and I think that changes the sessions, although it defines persistence of ip, since all the connections are of the dmz of the firewall, that is to say the same ip

Did you set up shared SOGoMailSpoolPath and NGMimeBuildMimeTempDirectory (e.g. via NFS) between all used SOGo instances?
I assume the second is not in your configuration.

I have the three servers with these variables that point to two folders in an nfs cluster:
SOGoMailSpoolPath = / mnt / spool;
NGImap4ConnectionStringSeparator = / mnt / temp;
 
but the problem continues, it fails to upload the attachments and when the email is sent, they remain in the drafts

NGImap4ConnectionStringSeparator should only "/" or ".", not a path.
You should set NGMimeBuildMimeTempDirectory to that path.

I have changed it as you say, I start balancing with round robin, same priorities in the three servers, in the first test when uploading 3 small pdf files, it already gives an error and begins to generate different drafts, one with the two files that has uploaded well and another draft with the file that failed.

Did you upload those files sequentially?
I mean, did you wait till one upload was done, before you started the next?

no, select three files at once

Then you probably have different memcached server for each of your server?

on each of the three servers in the sogo.conf SOGoMemcachedHost = 127.0.0.1
is not correct?

Your problem stems from using multiple uploads at once for one user.
Each file will open one connection in order to upload.
Those can potentially hit different servers.
Each server will not know if and what other servers get.
Therefore they only will see the other uploads, when they show up on NFS, which will take a while, because of caching.
Depending on file size, network speed and server load they will finish those uploads at different times.
Not knowing of the other uploads each server will assemble an email with only those attachment known to it.

In your example, you were lucky, as 2 uploads landed on the same server.

With only one memcached, those 3 servers can share information.
Therefore they will know of the other uploads and assemble the email accordingly.

Without that, you have to upload each file sequentially.

Configuration error - multiple servers require a shared spool path for uploads and also a shared memcached server.