Many mailbox providers look at the impression a sender gives over a period of time based on the quality of email that they send. Having a bad reputation will result in deliverability issues which can take time to recover from.
Sender reputation is gauged by analysing the behaviour of the sending domain name and IP address(es) over a period of time. The analysis is based on factors such as: Do recipients flag the email as spam? Is the sending volume consistent or erratic? Are they sending to a large number of invalid email addresses? Are they on any RBLs? Are they moving Email Service Providers (ESPs) frequently?
Get in touch if you need help with understanding or improving your sending reputation.