You say you have sent such large messages via Thunderbird and a web

interface which I find surprising for the facts mentioned by Martin but

it makes me wonder if an inadequate timeout setting in Pegasus Mail

could be breaking off the transmission before it has completed.

in mind that the timeout setting in the SMTP host configuration takes

precedence over the one in the General tab of Network configuration.

FWIW, 

at the office we routinely exchange photos and large files with clients and vendors.  Our experience is that delivery of messages larger than 20MB is unreliable.  An image resizer, a zip utility and Dropbox are all utilized as appropriate.

I use latest PM v4.63. I sent 15 photos to friend. But friend told he got only 8 photos in attachment and one of them he cannot open. I sent him all photos in zip. He told zip appeared damaged and cannot be open. Then I sent same zip but using Web-interface of same email box. He got zip successfully. After that I send same zip to myself using another email program (Mozilla Thunderbird). I have got zip without damage. Finally I sent same to myself zip by PM v4.63. And I received zip damaged. Now I am sure PM v4.63 cannot send large attachments. I use PM since 1996, but never had this problem. Even previous version of PM worked fine with attachments.

Make sure that the zip program is completely closed before trying to mail the zip file. Pegasus Mail needs dedicated access to the file.

Martin 

That was not a problem with zip (I use 7-zip). It was definitely closed. PM damaged separate .JPEG files too. I attached 15 photographs but PM sent only 8 and last was damaged.

Then I suggest you look at your AV software and exclude the directories that contain the files you want to send. I have not heard anyone else complaining about this problem, so it has to be localised on your machine. Sorry but it is impossible to troubleshoot a PC remotely.  The fact that some got through shows that it was something transient that occured.

Martin 

This does not relate to AV software or my machine. Other mail program (Mozilla Thunderbird) working on it sends all attachmens without problem. Even I sent anything using Web-based online mail services. The problem appears when I try send LARGE attachments in latest PM. When I tried send 15 photos they was about 8 MB each. Only 8 of 15 were attached and sent and latest attached was damaged. Then I tried send zip, it was about 120 MB - It was sent damaged with smaller size. In previous PM version I sent large attachments as well. But latest version damages large attachments. Again - all same files (15 large photos and zip including them) I successfully had sent with Thunderbird and Web-based services.

What you are describing is typical of an ISP that has a data size limit on size of messages. SMTP is not usually used to transfer 120 Mb files. Also it seems that because of the limit, the limit is likely bigger than 56 Mbs as the first 7 of 15 got sent ok, and the transmission stopped somewhere in the middle of 56-64 Mb total.. Probably 60 Mb limit has been set.  

It is up to the ISP to signal failure to send a message, back to the sender.

I suggest if you try each photo in a separate message, and they all arrives safely, that indicates that the messages are not "damaged" . 

Solution is to use another tool to share photos with others, such as Dropbox (especially the 120 Mb zip file). 

Martin