Outlook – How to Extract Email Addresses

Modern major mail services score spamminess based on multiple reputation factors, if available.

If the only factor is your IP and you don't mail often, this might be "the reputation of the /24 my IP is in". If you mail a lot, you might get your own reputation.

The easiest way to get reputation which "drowns out" the IP reputation of your netblock is to set up DKIM. Send your mails signed. This will establish a per-domain reputation, instead of per-IP reputation, and provides more signal to the automated scoring systems.

The more you work to help the recipients of your email figure out who you are and how trustworthy you are, the more signals they have to work with and the better a job the automated systems can do in deciding that you're legitimate, unlike whoever else bought a virtual machine in the same netblock, pumped out spam and then left, bills to hosting service unpaid and everyone else's reputation tarnished.

[some years later]: I wrote a blog post a couple of years back, walking through a lot of the steps needed to be able to more reliably send to large mail providers: https://bridge.grumpy-troll.org/2020/07/small-mailserver-bcp/

Outlook is very forgiving of formatting errors, and has been very good at introducing them. Try delivering them to a store where you can look at them in raw format. A local mailbox or Maildir store on the server would be ideal.

ContentHeaders may be checked by both desktop clients and webmail clients. Don't assume because it is webmail it supports HTML. Several webmail applications default to text mode due to security issues with HTML.

Ideally you should generate a multi-part MIME with both text and HTML format. This is the most flexible option. Properly done it should display well everywhere.