Another possibility is whether you have SSL turned on your SMTP settings for your provider. It is fairly common for email to be blocked when outside your ISP's network unless you have a secure SMTP connection. Like changing the default port number (what OB's link discusses), this is done to prevent non-customers from using the ISP's email for spam and/or other shady/illegal emails.
You can find the SMTP settings by going to the account's Advanced settings, and selecting SMTP, then your Primary Server. (description is for a Yahoo account, and may be slightly different for your ISP).
Also, as other's have suggested, you may have to talk to your ISP to get the exact setup needed.
Also, also, you may consider changing your primary email account to a third party email provider. Most ISP email accounts are POP3, which is less than ideal for mobile email. Third parties like Gmail, Yahoo, or even your iCloud account use IMAP. It also reduces your dependency on the ISP. If you should ever change ISP's (for a better deal, or if you move), you'll lose that email account. Besides the inconvenience of having to let everyone know your new email account, it can also cause problems your other online accounts. Many of them will be tied to your old email account as log on credentials, and as part of the verification process if you ever need to recover the password.
Even if it is not your primary email account, a second email account that is not dependent on your ISP is a good idea, and when allowed you should use it as an alternate address when setting up other online accounts.
