09-02-2011, 04:28 AM
I would guess that you may be triggering a throttle from whatever protection software is being run on the mail server. From my experience in working at an ISP with the NOC, they usually don't like to see large attachments being sent through email anymore than they like to see somebody try to send out to exceptionally large mailing lists. Email was really designed to transfer text messages and FTP was designed to transfer files.
What you may wish to consider doing is setting up an account, such as through Gmail, which you will use as your "workhorse" email account. Set up your email address (the reply address) to your "preferred" email account so replies go back to that account and then setup a forwarding on your workhorse account to your preferred account. This effectively makes your workhorse account a ghost account so you can stomp on their SMTP server to send those large files through; send large files through the workhorse account, it will throttle you but in the meantime you can send the letter emails through your preferred email account and get emails to your workhouse account without managing it separately.
No, recipients will not be able to tell you are using different SMTP servers unless they are parsing the headers on your emails. Providing of course that you again change the settings so they mirror your preferred account and the reply-to address is your preferred account. You can easily change the SMTP server in Mac Mail by clicking on the "From" field when you are composing a new email. This may cause some spam filters to possibly
flag your message as spam forcing the recipient to manually flag those emails as not spam. The reason for this is that if your email address domain does not match the domain of your SMTP server, it looks like spam.
Alternatively, if you are going to be consistently sending out large files to the same source then you may wish to discuss other options such as an FTP account or a web page you can upload the file through. I do not know about FTP programs on the Mac as I am still a fairly new full convert, but asking here will surely get a guru to give you some great programs to check out.
Hope this helps.