If you have a list of email addresses and want to send a customized email for each one you can use Google Apps Script to get emails and data from Google sheet and use a draft email in Gmail. Data in Google sheet can be used as variables, ie. {{email}} , {{user name}}, {{password}}
Here is a link with a ready-made code to use:
https://developers.google.com/apps-script/samples/automations/mail-merge