Possible Problem #1:
Your Email program is set to "never" delete the messages from the server.
That will cause the box to eventually fill up and then lock up.
You can login to your webmail account to confirm (go to webmail.whatever-your-domain-is.com - do not put "www" in the beginning). There you will input your username (the full email address) and password to login.
Then click "Mail" then "Inbox" at the top of the page you will see a "Quota Status" that will tell you how many MB used of how many MB available storage space you have. There isn't much because the email program (Outlook, Windows Mail, your Smart Phone, etc) is where the emails are permanently stored.
Free up some space by deleting all old emails in webmail and set your email program and/or phone to delete the emails after 5 days. Then restart your phone or email program and try sending an email to verify that fixed it.
Possible Problem #2:
If the mailbox is NOT full, Your settings have possibly been changed on your phone or computer due to an update it recently did for you automatically (thanks Apple or Microsoft!... sigh).
You just need to change it back to the correct settings. (See original settings in the email you received from us)
Possible Problem #3:
Your Internet service provider has changed a setting on their server to allow you to use port 25 (which is the standard email port). That just requires a phone call to your ISP (whoever your internet access company is) with either some major attitude because they blocked port 25, or a helpless damsel in distress that can't send emails out because someone blocked port 25 over there in the land of the inconsiderate. (you choose)
Test: Send an email from your email address using webmail (see login info and link above), then if you STILL can’t send through that, the problem is certainly a Phone call to me because it’s MY problem now. (insert damsel in distress comment here)
If you can successfully send an email out through Webmail the problem is (without a doubt) the Email Program/Phone’s settings or the Internet Service Provider.
If your settings are correct in your email program, then the final solution is to call your ISP. They will most likely attempt to dodge the task by saying the port is never blocked... “We don’t block port 25” but we’ve called well over a hundred ISPs over the years and shown them how to do their job MANY times in the past. What you always end having to do is tell them the port is no doubt blocked and your IT guy verified it through a "Port 25 Command line test". (This will shut them up IMMEDIATELY and get them to take it seriously and get off their butt and DO something about it).
I swear they have a big banner hanging up in the tech support office there that says... “Our Motto: Pass the Buck!” (yes, I know I'm slightly bitter about it. I just hate it when someone else's laziness causes headaches for the efficient and hardworking people of the world)
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