Have you updated any software recently?  This error also has happened to me after I upgraded a new antivirus, somehow it had affected my mail settings. What fixed it was when I re installed the email and set it up again. 

I cannot send email for over 24 hours now, but I can receive emails.
Whenever I send email, this error message appears:

SMTP Error Message:[*] Connection established to 121.54.64.27

>> 0035 220 smtp05.mrf.mail.rcn.net ESMTP

<< 0021 EHLO mum-s-computer

>> 0029 250-smtp05.mrf.mail.rcn.net

>> 0014 250-8BITMIME

>> 0019 250-SIZE 10485760

>> 0022 250-AUTH PLAIN LOGIN

>> 0022 250 AUTH=PLAIN LOGIN

<< 0012 AUTH LOGIN

>> 0018 334 VXNlcm5hbWU6

<< 0014 YW5uZWJyb3du

>> 0018 334 UGFzc3dvcmQ6

<< 0014 ZnJlZWRvbQ==

>> 0034 535 #5.7.0 Authentication failed

> I cannot send email for over 24 hours now, but I can receive
> emails.
>
> Whenever I send email, this error message appears:
>
> SMTP Error Message:
>
> Connection established to 121.54.64.27
> >> 0035 220 smtp05.mrf.mail.rcn.net ESMTP
> << 0021 EHLO mum-s-computer
> >> 0029 250-smtp05.mrf.mail.rcn.net
> >> 0014 250-8BITMIME
> >> 0019 250-SIZE 10485760
> >> 0022 250-AUTH PLAIN LOGIN
> >> 0022 250 AUTH=PLAIN LOGIN
> << 0012 AUTH LOGIN
> >> 0018 334 VXNlcm5hbWU6
> << 0014 YW5uZWJyb3du

You have just compromised a username and password.  When using AUTH LOGIN you are sending the username and password in base64 encoding and this is easy to decode.  You need to change these if they are the correct username and password for this account.

User name is annebrown

> >> 0018 334 UGFzc3dvcmQ6
> << 0014 ZnJlZWRvbQ==

Password is freedom

> >> 0034 535 #5.7.0 Authentication failed

That said, this is saying the username and/or password you are using is not correct for this host.

This happened to me before and a friend had me clean up my windows temporary files which fixed it. basically just empty trash bin, temporary internet files thing like that.