Oh no, one of my email accounts might be sending spam

Elaro

I'm running a virus check on my computer as we speak (Windows Defender), but one of my email accounts keeps receiving "Undelivered Mail Returned to Sender" messages from mailer programs on "gateway##.websitewelcome.com" where ## is a number. So either my account is sending spam and some of it is being blocked, or I'm being spammed with these messages. Because the messages that are "returned to sender" are definitely spam.

Hevach

Something to remember: Generating an email with your from header is trivial, all you need to know is the address. Spambots do this, they'll harvest known addresses to use in email headers rather than use a legitimate address. Sophisticated ones will even do it targeting your contacts list (or more sneakily, target you using your own contact list) because it becomes infinitely harder to filter them when they're coming from a trusted address that you don't want to block outright. Once your address is on a spam list (spoiler alert: they all are) there's no stopping that.

I'd go a bit more hardcore than just Windows Defender. Run Hijackthis and pay close attention to everything that runs at startup, particularly looking for things in weird places (like a second steam.exe that's buried deep in the userdata folder). Also download TDDSkiller to look for deeper rootkit infections that are invisible to most scans, if you're computer is on a botnet it's often a boot sector infection.

Shadowfire

Are you using an older version of Outlook? I just ran into an older malware that keeps coming up at random times that was hijacking a client's email account and making it send bullshit. Running Malwarebytes in safe mode was the easiest way to get rid of the thing.

If you're not using Outlook (or something similar like Thunderbird), I'd say Hevach is probably right that someone is just generating spam with your email address as the header. It's worthwhile to change your email password after doing the scans, though.

Elaro

I'm using Thunderbird, though.

BlindZenDriver

Your computer does not need to be infected for what you describe to happen.
Anyone can pretend to send e-mail from any address, that is no different that writing someone address in send form (or in the old days on an envelope). Simply put there is no systems that are immune to spammers doing that, some mails servers will pick up that something is fishy and block spam that uses a false/stolen e-mail address and it may lead to the address being black listed.
If you use the address to do more than receive non-essential newsletters and such, then I would suggest acquiring a new one just in case it gets blocked. Otherwise ignore it and maybe change those news subscriptions if you start missing them.