Cannot send mail. only receive, at home. fine at work

Question

I have 2 email addresses and both receive mail fine no matter which server I am using. However I cannot send outgoing mail from either address when I am at home. I CAN send mail at work or school, however, using their servers.
It gets worse though, because none of my roommates are having any problems with their mail! (But none of them have macs). So they can all send mail at home, but I cannot.
Any ideas?

Answer 1

Most, if not all ISPs used for connecting to the internet block the use of SMTP servers that are outside of their network on Port 25. Some ISPs allow the use of an authenticated SMTP server only that is outside of their network on Port 25 but some block its use regardless. Each ISP has different restrictions. These restrictions are in place as part of an overall effort to prevent spam emanating from the ISP's domain.
If an ISP is blocking the use SMTP servers that are outside of their network, it makes no difference if you are using an email client on a Windows PC or on a Mac.
Are you using the same or a different SMTP server for each account and if you are using a different SMTP server for each account, is one or both authenticated?
Who is your ISP used for connecting to the internet when at Home and do you have access to an email account and SMTP server that is provided by your ISP at Home?
Something to try first.
Go to Mail > Preferences > Accounts and under the Account Information tab for the account preferences at the SMTP server selection, select the Server Settings button below. Enter 587 in place of 25 in the Server Port field and when finished, select OK to save the changed setting.
Do this for each SMTP server if you use a different SMTP server for each account.
If this does not resolve the problem, you need to contact your ISP used for connecting to the internet when at Home to determine what is required to send messages with an SMTP server that is outside of their network or that isn't provided by the ISP.
If you have access to an email account and SMTP server provided by your ISP at Home, try adding the ISP's SMTP server to the Mail.app and select this SMTP server for each account which is invisible to all recipients.
When returning to work or school, you might be able to use this SMTP server for both accounts and if not, you must select the previous SMTP server or servers for each account.