Lot of Mac users faced this issue when sending an email in Mail app on Mac OS X High Sierra, Yosemite, Mavericks, and Mountain Lion. This problem will occur if SMTP server is messed with wrong accounts to use different servers.
- Open Mail app.
- Select Preferences.
- In Accounts tab, choose the account which is Outgoing Mail Server (SMTP) labeled as Offline.
- Click the SMTP server and choose Edit SMTP Server List from the list.
- Enter the right username and password.
- Quit Mail app.
- Go to Applications > Utilities > Keychain Access and delete all SMTP passwords from keychain.
- Enter the name of your SMTP server in keychain search box like mail.mailserver.com or smtp.mailserver.com.
- Search and find the entry which starts with smtp://. Copy and store the password in your hard drive in the different locations (for safety).
- Delete all relevant password entries for SMTP servers.
- Open Mail app and go to Preferences -> Accounts tab.
- Choose your account SMTP server from Edit SMTP servers list.
- Re-enter the password that you just deleted.
- Go to Apple menu -> Force Quit.
- Choose Mail app and select Force Quit.
- Restart your computer.
- Open Mail app and check your issue.
Go to Network Preferences -> Advanced section of Ethernet and WiFi interfaces. Add the following DNS addresses.
- Go to Mail Preferences.
- From the left side list, select the account which causes the issue.
- Choose “Edit SMTP Server List” from the outgoing mail server drop-down menu.
- Correct the username by appending domain name 9Ex: @gmail.com or your own domain).
- Re-enter the password.
If you knew any other solutions for SMTP server offline Mac mail issue, inform us via comments.