Continuing the discussion from Emails are stuck in outbox:
I looked in settings, general, services but there is nothing that says Airsync.
So now what? Can receive but not send messages.
I sent a big charity email out in a hurry so think that is why it got blocked.
Can receive incoming but nothing will send out.
I looked in settings, general, services but there is nothing that says Airsync
Some SMTP mail servers don’t use AirSync so that can be why you may not have that option showing. MS Outlook.com, Live.com, Hotmail.com accts “do have AirSync” and don’t use the normal SMTP Server Tab settings. So all depends on what SMTP mail server you use.
If you can’t send to any email address, then update what type of mail account you have eg: POP, IMAP, Exchange, Office 365 or iCloud, an also “who is your mailbox provider”.
I sent a big charity email out in a hurry so think that is why it got blocked
Looking up that error on the Microsoft website and other websites, 5.1.8 sending error is normally because eg: “the receiving server has blocked your email address” due to being suspected of sending spam and no longer allowed to send mail.
This might have happened due to eg: you may have been sending “too many emails at a time to that email address” where the receiving server has a limit. Or “something in the emails looks like spam” like in the charity email you mentioned, and the receiving mail server then blocks you.
So then the only way to email that email address again “if you still get the same sending 5.1.8 problem” within eg: 24-48 hours , would be to contact the receiving mail server technical support and ask them to “remove you from their blocklist” or have the receiving person “contact their mail server technical support their end”, and ask to have you removed from their blocklist.
The other alternate is to send to that specific email address using an alternate email address.