I use one central email address: stephen@gmail.com (email anonymized). I want to add my work address stephen@fake.company.com to my primary address so I can control it from there.
Receiving
option1: gmail's pop3 importer
Gmail cannot use IMAP to download messages (or I haven't found out how) from other mail servers. It can use POP3:
option2: forwarding
What i prefer is to make the mail server at fake.company.com to forward all the messages to stephen@gmail.com. This depends from server to server.
Sending
If you want to pose as stephen@fake.company.com from stephen@gmail.com, you have to use fake.company's email servers, here's how it's done:
On stephen@gmail.com's settings, do:
Gmail -> settings -> accounts -> Add another email address you own
Then add the info of stephen@fake.company.com:
NOTE: Uncheck "Treat as an alias"
Then add the server info:
Then an email with a code will be sent to stephen@fake.company.com
Give the code to gmail:
Now we're ready, compose a new email and put a different FROM address:
Now when you receive an email to stephen@gmail.com designated for stephen@fake.company.com and you click reply, gmail will automatically put stephen@fake.company.com as the FROM address.
NOTE: People on the other side will never know you use stephen@gmail.com.