Server-time-out usually means that the sending and receiving mail server are busy and having different timeouts when one of them gave-up while the other is still trying, resulting in the connection being terminated unexpectedly.

It is possible that you have exceeded the time or quantity of email sent, because most ISPs limit their user's access to their mail server by time or by the quantity of emails that can be sent per login, busy, firewall, etc.

Many ISP mail servers reduced the number of concurrent connection--meaning that; although your ISP mail server is accepting your emails and relaying them to the recipients, they may not be delivered or accepted by the intended recipient ISP mail server due to various reasons like: busy, anti-virus firewall, etc.

For example:
You have sent 500 AOL domain emails to your ISP mail server and were being relayed, but due to; concurrent connection--AOL mail server have reject some or all of them because AOL does not allow your ISP mail server or your PCs mail server to exceed sending the maximum number of emails and/or by time limit imposed per connection.

All of them?
See: Valid AOL, Yahoo, Hotmail etc., yet my email bounced

Undeliverable emails do not necessarily means that they are invalid.

Try submitting a few messages to that domain at a time and keep increasing the number until you find the maximum quantity or time limit--accepted by the server, or avoid broadcasting emails during "peak" hours or do not group all your mails with the same email domain address under a single list.

For example: Don't group all your email addresses; AOL's, Hotmail, Yahoomail, etc., if the your mail broadcasting program automatically does that for you and nothing can be done about it, then you should look for another program like mailsbroadcast--our program send out email addresses randomly, therefore avoid spamming any single mail server.

If you are using your own mail server and mails are rejected with can't be delivered with notification like; server-time-out.
See: Mail Server FAQs