Sender reputation is the score mailbox providers like Gmail and Outlook keep for every sending domain and IP address. It decides whether your email lands in the inbox, the spam folder or nowhere.
It's shaped by how many emails bounce, how many people mark you as spam, whether people reply, and how long and consistently the domain has sent mail. New domains start with no reputation, which is why they need warming up.
For outreach, protect it with verified addresses, low daily volume, plain text and relevant emails people don't want to report.
Example
Margin's outreach domain keeps bounces under 2% and sends 20 personal emails a day, so its reputation stays healthy.
Putting it into practice
Where to use it: Email deliverability for link outreach and Mailbox warmup calculator. For the longer version, read the limits and risks of sending outreach from Google Workspace.