The issue about SPAM is a fairly complex 'what do I need to do' conversation.
When Doug G says
Assuming your smtp server is properly setup your mail should be more reliable when you send via the smtp server.
He refers in part to the need to ensure that your smtp hostname banner matches your email domain.
Some SMTP servers will reject mail if the reverse DNS doesn't match the HELO/EHLO hostname used in the connection. If your mail server's hostname is mail.mydomain.com then your reverse DNS, MX record, HELO/EHLO, and SMTP greeting banner should all be mail.mydomain.com as well.
SPF records set in the DNS servers in some companies to verify the IP space you send from. Companies that use this often deprioritize or mark as SPAM records that fail this SPF-enabled domain check.
Then there is the volume, rate, and content of your email. Each play a part in how other mail servers and your ISP determine SPAM. Too high of volume then your ISP will blacklist your SMTP IP and Hostname. Same if your rate is too fast. You also need to worry about the typical content that these systems filter.
Your ISP will not tell you the rate or limit that you can safely use, as the SPAMMERS will use this information to cleverly send SPAM within their rules. So your best to be conservative.
On mail package that gives you fantastic control over how much and how fast is SwiftMailer.
You are best to group large batches of email in 30 - 40 messages at one time with 10 minutes wait between the next queued batch.
Hope this helps clarify some of the main issues.