It's important to find the right frequency for your campaigns. The best guidelines are:
- No more frequent than once a month.
- No less frequent that once a quarter.
Sending too frequently can result in list fatigue. What is list fatigue and how to combat it.
Sending too infrequently can result in higher-than-normal spam complaints. If it's been a long time since the recipient last heard from you (or since they signed up for your list), odds are they have forgotten giving you permission to email them. Best practice is to remove anyone from your list that has not engaged with you (i.e. opened an email, purchased from you, or signed up for your list) in at least a year.